Are Modern Humans and Neanderthals the Same Species?

 No, yes, maybe.

A species is typically defined as  a population, or group of populations, that can be recognized and distinguished from other such populations by a distinctive set of characteristics, and which is reproductively isolated from other such populations. 

Whether two populations of related organisms are classified as belonging to a single species or to two separate species is a decision made by taxonomists depending on two things: the degree to which the entities  can be distinguished from one another, and/or the degree to which they are reproductively isolated from one another. Fossils of modern humans, Neanderthals, and a third group known as Denisovians, are clearly recognized and distinguishable from one another and so are considered separate species by anthropologists.  This is consistent with how fossils in general are classified, as we typically have little or no information on reproductive isolation among them.

The growing evidence of genetic exchange between Neanderthals,  Denisovians, and modern humans some 40,000-60,000 years ago, however, suggests a possible alternate taxonomic decision - that they were all part of the same species. Though only modern humans remain today, the three groups overlapped  geographically for roughly 20,000 years after modern humans migrated into Eurasia. Genetic analysis confirms that all peoples of Eurasia possess between 1-4% Neanderthal DNA in their genetic makeup.

We know far less about Denisovians, who inhabited eastern Eurasia, but evidence is mounting that they too exchanged genes with modern humans, with their DNA now detected in indigenous human populations from central Asia to Papua New Guinea and Australia.  

The exchange and retention of Neanderthal DNA likely enhanced the ability of human immigrants from Africa to adapt to the colder climates of Eurasia. The February issue of National Geographic provides a fine review of our current knowledge of the human family and the interactions among the species. The article incudes a fascinating illustration, somewhat speculative of course, of a peaceful family scene of a Neanderthal man, his modern human wife, and their infant son huddled around a campfire. It hardly seems more remarkable than an interracial family today. 

Since they were not reproductively isolated from one another during that period, the three species of  humans might reasonably be combined into  a single species, and recognized as three subspecies. 

I boldfaced the word "decision" at the beginning of this post because classification is a human artifice, a matter of convenience for discussion and recording of information. Names, and taxonomic ranks are not a part of nature.  Taxonomists may not agree on how distinct a species must be from other species, and how complete the reproductive isolation must be. The category of species is of higher rank than subspecies, and therefore indicates a higher level of  character distinction and reproductive isolation. The history of plant taxonomy is full of conflicts between "lumpers" and "splitters," who disagreed on how broad species should be. Splitters recognized greater numbers of more narrowly-defined species, while lumpers were more tolerant of variation within species and recognized fewer species. 

Our expanding understanding of reproductive isolation has generally favored lumping, as we realized that apparently different species of plants or animals were mere variants of large, interbreeding populations. For example, the World Checklist of Vascular Plants currently recognizes 437,645 species, and over a million synonyms - formerly recognized species that have been lumped into the recognized species. As a more specific example, 46 species of the genus Rubus (blackberries and their relatives) are now recognized from South America, out of 110 that had been named by earlier taxonomists.

 Incidentally, I chose to use the human family as the focus of this discussion because it has an unusually extensive fossil record, as well solid evidence of genetic exchange among species, including some that are now extinct. Adding the time perspective illustrates the source of many taxonomic problems. We rarely have fossils of ancestral plants, and so plant taxonomy deals almost exclusively with present day plant populations. In theory, however, everything I've said and will say in this post applies to plants as well. 

Reproductive isolation prevents two species from sharing genes and blending back together, and so each species continues to develop its own characteristics. Reproductive isolation is a tricky factor, however. Closely related plant species can often hybridize. The list of garden plants that are of hybrid origin is extensive. In nature, however, closely related species don't normally interbreed. They are isolated geographically, by ecological preference, or in many plants (such as orchids) by pollinator specificity, all of which can be overcome by plant breeders. 

A liger at Jungle Island, Miami. Lions
and tigers naturally live in different
habitats and so rarely meet. If they do
happen to meet and interbreed, the
resulting offspring are unable to 
produce viable sperm or egg due
to chromosomal incompatibilities, and
so do not produce further generations.
The genetic integrity of the two species
thus remains intact. 
Photo by Maxitup16, CC by SA 3.0

 Closely related species may interbreed in nature where their geographical or ecological boundaries meet, but the resulting hybrid individuals are often less fit for survival outside of the narrow interface zone, and the alleles of hybrid origin usually do not spread back into the parent populations. So despite occasional hybridization, the parent species remain effectively isolated genetically. Animals like lions and tigers, or horses and donkeys, can mate upon rare natural encounters, or in the hands of zookeepers, but their offspring are sterile, revealing a more complete separation of species. 

So lions and tigers are unequivocally different species despite the occasional hybridization. Not so with the three human species that interbred in Asia. Hybrid individuals were able to reproduce and spread hybrid gene combinations throughout the parent populations. 

We can say that lions and tigers are further along in the biological process of speciation than the three species of humans were. Speciation is the real process of populations changing over time, in contrast with the arbitrary human process of classification. As species drift apart over time, chromosomal rearrangements arise that reinforce the tentative reproductive isolation caused by geographical separation, making further genetic exchange impossible. The ambiguities of this situation, and classification in general, are due in large part to the fact that groups of related species that we encounter in nature are at different stages of speciation. Some are recently separated and still capable of interbreeding, while others have become chromosomally incompatible and fully separated. 

 All species begin as a population that splits off of from a pre-existing species.  The Neanderthal/Denisovian lineage split off from an ancestral species in Africa that was also ancestral to modern Homo sapiens. So at the beginning, they were the same species. Members of one lineage later migrated to Eurasia, where they split into what we now recognize as H. neanderthalensis and H. densoviensis. As these Eurasian populations adapted to the very different environments they encountered they developed distinctive characteristics, but not solid reproductive isolation from their African cousins that would migrate later. If the three species had remained separate for another 100,000 year or more, they might have crossed the line into fuller genetic incompatibility, and then without doubt would be separate species.

Before that stage is reached, however, species may exchange genetic information temporarily or possibly merge back together, combining the best of each, and survive as an improved, hybrid population. When immigrants from Africa met their Eurasian cousins, something in-between happened. Genes acquired from Neanderthals for lighter skin, hair, and eye color, along with other things, helped those immigrants adapt to the harsher conditions they encountered. The outcome was apparently not so good for Neanderthals and Denisovians. For reasons still not fully understood, these groups became extinct soon after.

So the answer to our title question is still a matter of taxonomic opinion. The speciation process was not complete with respect to reproductive isolation, and so were the physical differences enough to warrant speciation? The outcome is of interest on many levels. If, hypothetically, a group of Neanderthal survivors were found today, would they be granted all the civil and religious rights we delegate to ourselves? For biology students, anyway, it is a vivid illustration of the speciation process, the dynamic nature of evolution, and the tenuous authority of formal taxonomy.

Incidentally, the very first member of the genus Homo, recognized by dint of some distinctly human characteristics, logically must have evolved from ancestors formally classified in a pre-existing genus. It is clear now that that genus was Australopithecus, which existed for several million years before Homo arose. That, by definition, makes Australopithecus a paraphyletic genus, something prohibited in clade-based phylogenetic taxonomy. The rule is that all formal taxa must be monophyletic - complete clades consisting of a common ancestor and all its descendants.  Anthropologists at first went through considerable taxonomic acrobatics to resolve this problem, splitting Australopithecus into ever finer monophyletic units. But in the end, there was no escaping the fact that, unless you believe in special creation, the first humans must have descended from something not quite human. 

[BTW - the answer to the old chicken or egg question is clear. The very first chicken hatched from an egg laid by an almost-chicken!]

Because of the need to provide formal names (binomials consisting of the genus name and the specific epithet) for the Australopithecines, paleontologists have come to relax the rule to greater or lesser extent. However, one must acknowledge that every genus ever identified by taxonomists logically began with a common ancestor that emerged from a pre-existing genus. The fact that we don't have fossils of that pre-existing genus, as we generally don't with plants, does not negate that inescapable conclusion. One proposed solution is to abandon formal taxonomic ranks like the genus, and simply name clades, but then it becomes very difficult to provide formal names for organisms. This is something I have been obsessed with throughout this blog series, and you can review my earlier attempts to clarify the situation:

The great botanical butter battle book (30 Aug 2012)
Making the ancestor problem go away (18 Oct 2012)
Minding your stems and crowns (3 Jun 2015)

The Problem with Wookiees

Photo by Scott Ruether,
CC BY 2.0 

When making up alien creatures, imagination has no bounds. Alien characters in sci-fi stories are most commonly human-like, but with animal features. But be they scary, funny, or loyal comrades-in-arms, they exist purely for entertainment. We might laugh at the absurdity of a giant slug-like creature with a human face slithering around on a desert planet, but we don't care when engrossed in the story.

Nevertheless, the critique of aliens is a great exercise for biology students.  The fundamental rule is that evolution can only proceed through logical steps. Every feature of an alien needs to be explained in terms of adaptation to environmental conditions. In terms of this purely academic exercise, my favorite aliens, the Wookiees, raise a number of red flags.

I recently discovered a webpage purporting to describe the biology and evolution of Wookiees on their home world of Kashyyyk. Whether the facts stated therein were created by the writers of the Star Wars series or made up by imaginative fans, I don't know. But since they have been posted, they are fair game. As a botanist and biology teacher, I am obligated to respond! 

Wookiees qualify as human-like sentient beings. They are smart enough, and have the upright posture, flexible shoulder joints and grasping hands needed to manipulate weapons and fly spaceships. So their evolutionary history must account for both their body anatomy and the development of their large brains. We assume they acquired their human-like traits through an evolutionary history similar to ours. Indeed, they are described as tree-dwelling primates.

For our own species, however, the path to humanity required both an arboreal (tree-dwelling) phase and a hunting/gathering savanna phase. The arboreal phase evolved in tropical rainforests, which provided a rich diet of flowers, fruits, seeds, leafy shoots, and insects or other small animals. Accessing these food sources required efficient mobility to move around a tree canopy and from one tree to another. The interlaced system of slender branches and vines of the tropical rain forest fostered such mobility, resulting in grasping hands and flexible shoulder joints. 

The hands of our ancestors were uniquely claw-free. Claws as found in other animals, were replaced in our arboreal ancestors by flat fingernails and soft, sensitive undersides, allowed them to grasp relatively slender branches and vines firmly as they swung, ambled, and leapt about. They evolved uniquely twistable,  double-boned forearms that could turn the hands upward or downward, aiding in locomotion, as well as for reaching, picking, and manipulating food items.   

Shoulders could rotate, allowing us to reach in multiple directions for food items, or branches by which to "swing through the trees with the greatest of ease." Such anatomical features would later be essential for wielding sticks and stones, for playing baseball and ultimately assembling watches and cell phones. So among the denizens of the earth only primates have the dexterity to play baseball. Though it might be a nightmare to keep them focused on the game, a match between between the Chicago Chimps and the Green Bay Gorillas is at least technically possible, because primates have arms and hands capable of throwing things. I've seen annoyed gorillas in a zoo throw something much nastier than a baseball at heckling tourists. Non-arboreal animals are even more severely limited. Shoulder joints and paws (or hooves) are rigidly limited to their walking/running function. Though squirrels, cats, and raccoons can hold food items with their front paws, they cannot grasp, and cannot throw rocks. 

The second phase of human evolution came in the radically different, wide-open vegetation of the African savanna, where we had to make a leap in intelligence and brain size.  Water and food resources in the savanna were widely scattered and highly seasonal and we had to remember the places where they could be found. The fruits, vegetables, and occasional beetle grub so abundant in the rainforest were too scarce in the savanna to sustain us, and so we had turn to hunting small animals, who generally did their best to avoid becoming our dinner. We did not have the strength, speed, claws, or toothy jaws of our competitors, and so could only survive on our wits. We had to learn to use sticks and stones to catch and kill prey, as well as to defend ourselves against the bullies further up the food chain who considered us as slow-moving food items. 

Sticks and stones led to bows and arrows, houses, wheels, and ultimately space ships. Our brains grew to human proportions and capabilities as we adapted to the harsh realities of the savanna, but would not have been possible without the flexible anatomy we inherited from our arboreal ancestors.  

Koalas are arboreal animals with claws and some
grasping ability. As slow-moving leaf-eaters, 
there was less selective pressure for greater agility
and mobility, not to mention for intelligence. 
Photo from GreenLeft.org

Red flag #1: Though Wookiees appear to have grasping hands and rotatable arms, they are also said to have retractable claws for climbing up trees. Claws big enough to support the considerable weight of an average Wookiee adult could not be retracted enough for delicate work like pressing the triggers of weapons, operating the console of a space ship, or using tools to repair a space ship. In clawed animals, bone structure and musculature are focused on supporting the claws, which must bear the weight of the animal.  There are arboreal animals that climb with claws: squirrels, sloths, koalas, etc., but of these, only squirrels are really agile in trees. They, however, are small and light-weight, depending more leaping rather than grasping and swinging to move through a forest. The others are far more limited in mobility and depend on more limited diets. 

Red flag #2: The forests of Kashyyyk are said to be dominated by coniferous Wroshyr trees, sounding more like a boreal forest than a tropical rain forest, and would provide neither the variety of food sources nor the 3-dimensional jungle gym structure to move around in.  Also, Wookiee ancestors are said to have been carnivores. However, all known arboreal animals are vegetarian or omnivorous. It's too difficult to run down prey larger than insects, frogs or lizards in the forest canopy, unless you're a bird of prey, So it is unlikely that Wroshyr forests could have fostered the evolution of the flexible anatomy required for the later development of technology.  

Wroshyr trees are also said to be massive, supporting Wookiee communities within their trunks and large branches. Such trees are said to average 300-400 meters in height, with some varieties as tall as several kilometers. The tallest trees on Earth reach a little more than 120 meters, and are pushing the limit of the physical force of transpiration to lift water against the pull of gravity. If gravity were a little less on this planet, trees might be a bit taller, but if gravity were substantially weaker, the planet could no longer hold onto its liquid water or atmosphere. So  trees significantly larger than those on Earth are highly unlikely.

Red flag #3. It appears that Wookiees stayed in the forests. They did not face the challenges of the savanna or any other environment that could drive the evolution of upright posture and higher intelligence. Why is this important? If they indeed lived continuously in forests, they would have advanced little more than the great apes of Earth. The forests did not provide the necessary challenges.

Red flag #4: The dense hairy coat of the Wookiees, while fine for primates who never left the rain forest, would be a real liability in the savannas, in particular for developing greater intelligence, i.e. bigger brains. The brain is the most heat-generating organ of the body. As it increased in size, we required an improved cooling system. To provide cooling surface for all that brain heat, along with heat from the scorching savanna sun, we lost body fur and enriched our bare skin with fine sub-surface capillaries and  a high density of sweat glands. We retained hair on top of our heads to protect our brains from direct heating, but the rest of the body was freed up for cooling the blood. 

As a unique, omnivorous species in the savanna, we could not nap in the shade of trees after a big kill, because hunting and gathering had to continue all day long. So we became "naked apes." This is nicely explained in a 2010 article in Scientific American, by Nina G. Jablonski. Being naked therefore was also essential for breaking through to human-level intelligence. This also applies, incidentally, to our homegrown hairy aliens, Sasquatch and Yeti.

So we are left to ponder how the Wookiees became intelligent beings. If they did not follow the human game plan, what evolutionary history did they have? I keep coming back to my conviction that if we were to discover intelligent, technologically capable, alien life forms, they would have had to go through a similar evolutionary pathway as ours, and would look boringly like us (see my post on the inevitability of humans. Even with very human-like aliens like Vulcans, we have to explain pointy ears, green blood, and internal organs that are somehow different. If you are a teacher, try this with your students.  It's a topic that's guaranteed to wake up that guy in the third row who's been sleeping since the lecture on Cyanobacteria. Oh, and less you wonder if technological aliens could have evolved from sea creatures, have you ever tried to throw a baseball under water?

Kicking the plastic habit 2 - What about Silicone or Rubber?

 Kicking the plastic habit - What about Silicone or Rubber?

In my previous post, I neglected to mention two other materials that might replace some plastic use - silicone and rubber. Are they any better?

The short answer is yes, but with some important qualifications. Neither is made from fossil fuels, and so that is a plus.  Both are less toxic to us and our environment, and neither breaks down into fine particles like microplastics. 'll deal with silicone first.

Silicone is a rubber-like material that is actually made from plain old silicate sand (SO2), as is glass. There are many silicone products that could replace fossil fuel-based plastics, including multiple-use stretchable bowl covers that can replace plastic wrap for saving dinner leftovers. There are also silicone baby bottles, water bottles, mugs, etc. not to mention spatulas and other utensils that take advantage of silicone's high heat resistance. So, sounds like a pretty good deal, right?

Maybe. A significant factor in evaluating any technology is the degree of processing involved. A longer, multi-step manufacturing process is likely to require more energy and result in more pollution. Cutting a piece of bamboo culm to use as a drinking cup is far more environmentally friendly than extracting the cellulose from the bamboo and then using it to make socks (see the process for making viscose in my previous post. 

Glass is essentially sand that has been melted and re-shaped. It takes a little heat, but is a simple process that has been used for centuries. Silicone, however, is a much more radical transformation of sand. Like carbon, silicon atoms have four bonding sites, so have a similar ability to form long polymers with varied side chains.  Intense heat (around 1000 degrees Celsius) is required, however, to remove the oxygen from silica molecules, transforming it into pure silicon.  The silicon is further processed, with more heat, to make it form plastic-like polymers.  The heat required is generally derived from fossil fuels. 

The amount of energy required to make one kilogram of silicone is roughly11 kilowatt hours, the equivalent of 5 kg of coal, or nearly a gallon of petroleum. Roughly the same amount of fossil fuel is needed to make a kilogram of plastic. So the consumption of fossil fuels is about the same for silicone as it is for plastics. Making silicone puts carbon dioxide into the air, while making plastics solidifies it into the stuff that litters the oceans and our water supply. Both problems can be addressed, but the track record so far is not good. Once again, one must look for responsible producers when considering silicone products.

The starting ingredient for both glass and silicone is abundant, but the mining of sand, potentially defacing the landscape and disrupting local ecosystems, is a factor to be considered in the eco-friendly equation. Glass is a better use of sand, and can be recycled over and over. It appears that silicone is non-biodegradable, or at least takes a very long time to break down. So, like plastics, it must be safely buried at the end of its useful life. In the end, silicone has an edge over plastics in that it is less toxic and does not break down into microplastics. 

In the rubber tree, Hevea brasiliensis, latex
drips freely from cuts in the bark, a process
called tapping, which does not harm the tree. 
Photo by Vis M, CC BY-SA 4.0.

What about real rubber? Natural rubber has the potential to do a lot of what silicone and plastics do, but is plant-based and biodegradable. Rubber comes from the sticky latex formed by certain trees and herbaceous plants.
The function of latex for the plants themselves is generally said to be as defense against insect damage. 

The primary commercial source  of latex is the rubber tree, Hevea brasiliense, in the Euphorbiaceae (Spurge or Poinsettia family) but many  members of the Fig Family, (Moraceae), also produce latex, as do members of the Milkweed Family, and others. Dandelions (Asteraceae, or the Sunflower Family) were grown during World War II as an alternate source of latex, when America and its allies lost access to the commercial rubber plantations of Southeast Asia. The cultivation of dandelions for latex is being revived as a more environmentally-friendly alternative to rubber plantations.

Rubber from natural latex sources can replace plastics and silicone in many applications, and is biodegradable, though depending on how it has been processed it may take many years to decompose. Rubber that has been vulcanized to make it more durable, breaks down much more slowly than raw latex or minimally processed rubber.  There is experimentation with bacteria that can speed up the decomposition of rubber. Of course, synthetic rubber is a different story altogether, as it is made, like plastic, from fossil fuels. 

Processing latex in the field and conversion to rubber products in factories are, of course, potential sources of pollution. There are ways to minimize such problems, and consumers can choose products that have been responsibly manufactured.

The biggest potential problem with rubber is that the rubber trees are generally grown in massive plantations, resulting in the usual disease and pest problems associated with monocultures, along with   deforestation and the loss of biodiversity. Similar problems plague palm oil plantations, and indeed most modern agriculture. The cutting down of biodiverse rain forests for rubber or oil production, seems somehow more egregious than replacing grasslands with wheat or corn. 

A better way to grow rubber is through mixed plantings with other crops. It is compatible with growing other tree species for wood, cellulose, fruits or nuts, or for intermixing with low-growing shade-tolerant crops. Such farming can be as profitable as monoculture plantations. It is similar to how indigenous peoples originally harvested rubber directly from wild-growing trees in the forest. 

Bottom line, silicone, and even better, natural rubber, can be considered options for limiting our use of plastics derived from petrochemicals, if the energy and environmental impacts can be mitigated. Once again, with so many people on the planet, there is a cost to whatever technology we use. We can only choose the lesser of evils. 

Kicking the plastic habit with a little help from plants

 I was recently on the expedition class Viking Octantis for a cruise along the Chilean coast. The ship staff included a team of scientists and naturalists who were actively investigating environmental changes in the oceans while engaging and educating the ships passengers.  One lecture dealt with the issue of plastics in the oceans, which now pollute even Antarctic waters and coastlines. That sobering lecture was the inspiration for this essay.

We live in a plastic world. Plastics are durable, inexpensive, and convenient. From plastic water bottles, packaging, and shopping bags to durable kitchen gadgets and automobile parts, plastic products derived from fossil fuels have made life more convenient and less expensive. HOWEVER, from start to finish - from the extraction of fossil fuels to the manufacture of plastic products to their eventual disposal -  the use of plastics poses serious risks to the environment, wildlife, and our own health. 

A sea turtle entangled in a fishing 
net. Photo by Doug Helton.

The primary problem with plastics is that they never go away. Plastic production accounts for about 6% of fossil fuel use, a small amount compared to what is consumed for automobile, aircraft, and industrial fuels, but because of their near indestructibility, the volume of plastics on our planet builds up year after year.  According to a recent study (Li et al., 2023) the amount of plastics produced annually (as of 2019) was a staggering  460 million tons, of which only 9% is recycled. 

The remains of an albatross with stomach full
of plastic debris. photo by Forest & Kim Starr (USGS)

Regrettably, much of this plastic ends up in the oceans, where it can persist for hundreds of years. In 2014 there was one pound of plastics for every five pounds of fish in the oceans worldwide. Plastic water bottles and conventional disposable diapers can last up to 450 years in the sea. Marine mammals, sea turtles and birds get tangled in the debris and drown, or starve to death when their guts get clogged with plastic debris. 

Microplastics are everywhere.

But that's only part of it. While chemically indestructible, much of the plastic debris eventually breaks down into finer and finer particles, what are referred to as microplastics. These fine particles can be taken up by filter-feeding plankton, shrimp, and krill, and then move up the food chain.  Ongoing research is showing that microplastics in our food and water can harm human health (Li et al., 2023).

The major synthetic fabrics used today, polyester, nylon, and acrylic, are highly valued for their smooth feel, resistance to wrinkling, and quick-drying properties,  In addition to their eventual arrival at a landfill, every time such fabrics are washed in your home washing machine, tiny bits of fiber break off and add to the microplastic load in our water supply. 

Cleaning up the mess that is already there, and proper disposal of future plastic waste represent a huge challenge.  There is a glimmer of hope with the discovery of bacteria that can digest plastic. They potentially could help with disposal and the cleanup effort, but so far have not been used on a practical scale. 

Silkworms feed on mulberry leaves and excrete silk
filaments to make a cocoon. This natural process is
analogous to making viscose, but without the harsh
chemicals!
Photo cc Fastily at English Wikipedia

To slow the accumulation of plastics, and maybe even stop it, however, we can turn to natural biodegradable alternatives. Animal products, like wool, leather, etc., can take up some of the slack, but plants can provide  an even more massive source of organic building blocks to replace fossil fuels. The unique plant compound  cellulose, which strengthens cell walls, fibers, and wood, can effectively replace fossil fuel feedstocks in almost any application. Plants have always long provided us  with natural fibers from cotton, linen, and silk (the latter with a little help from a  caterpillar!). If we can shift back more to these natural fabrics, we can make a big difference.

Other cellulose-based products that have been with us for over a century are plastic-like celluloid, cellophane, and rayon.  Celluloid was formerly used to make billiard balls and other solid objects, as well as movie film. Its use was discontinued due to its unfortunate tendency to explode or catch fire upon impact. Cellophane and  rayon, continue to be in use, but have their own associated hazards.     They are made by treating cellulose with harsh chemicals to make a mush that can be extruded through tiny pores to slender soft fibers, or through thin slits to make cellophane sheets. Fabric fibers made in this way are called viscose. The handling and disposal of the chemicals used in this process are bio-unfriendly, if not done carefully. Before purchasing such products, including those puzzlingly soft bamboo socks, one should check whether the manufacturer is environmentally responsible.  

Modern bioplastics have been devised to avoid these problems. One cleaner process converts the plant cellulose into lycocells, similar to viscose, but using less chemicals. Tencel is a product made in this way. There are now many commercial ventures starting to manufacture environmentally-friendly, plant-based bioplastics including styrofoam-like materials, other packaging materials, and even biodegradable disposable diapers. If these catch on, we can significantly shift away from plastics. 

Bamboos are the poster child for fast-growing 
sources of cellulose. New stems are continuously
produced from underground rhizomes, and can
grow up to 3 feet per day.

The cellulose for these processes can be extracted from a variety of sources,  Fast-growing trees, like Eucalyptus, Casuarina, Populus, and Paulownia, along with bamboo and sugar cane stalks, can be grown like other agricultural crops, as a source of cellulose. 

Eucalyptus harvest in Cameroun. Eucalyptus  is
native to Australia, but has been widely planted in
warm parts of the world as a source of wood and cellulose.
It unfortunately often becomes invasive, displacing native 
vegetation. Photo by Jean-Louis Heckly, cc Wikipedia
commons/

Cellulose, of course comes in premade forms like wood and bamboo. Wood can be carved into almost any shape: spoons, spatulas, bowls cups, toys, toothbrushes, etc., for which cheaper plastics are often used today. In traditional and modern technology, bamboo culms can be cut and used directly for housing, flooring, scaffolding, water pipes, containers, paper, and hundreds of other structural materials. 

The drawback to wider use of cultivated bamboo and other cellulose sources is that it requires more land to be converted into agriculture, diminishing the biodiversity of natural plant communities, and creating disease and pest-prone, fertilizer and water consuming monocultures. 

With so many people on the planet, there is no technology that is without environmental cost, but overall, cultivating biodegradable cellulose sources, if done responsible, is far better than continuing to load our oceans and drinking water with plastics. 

The plastic problem seems overwhelming, and much of the solution will have to come from government legislation and reform of manufacturing processes, but individuals can take small steps that will make a difference.  Can you commit to one or more of the following?

1. Use less clothing made of synthetic fabrics and go back to cotton and other natural plant based fabrics. Use an iron if wrinkles offend you!

2. Support emerging bio-friendly technology, including environmentally-friendly diapers, whenever available to replace plastics. 

3. Trade in your plastic toothbrush for one made of wood and natural fibers.

4. Use glass, stainless steel, wood, or bamboo-based kitchen utensils, bowls, storage containers, etc. instead of plastic ones. 

5. Filter your own water and use glass or stainless steel containers for carrying it around. 

Cactus? Look again!

Aloe erinacea superficially resembles a 
cactus, but closer examination reveals 
that the plant consists of closely-spaced,
spirally-arranged succulent leaves with
spines along the edges. In cacti, leaves 
are done away with altogether, or adapted
as spines.

 The picture at the right is a member of the genus Aloe. We all know the most common member of this genus, Aloe vera,  grown as a garden ornamental, as a source of skin ointment, or for its edible leaves. The species pictured, Aloe erinacea, with its compact, rounded overall shape and prominent spines, superficially resembles a small barrel cactus. Aloes and cacti are both succulent plants adapted to survive arid conditions by storing water in their tissues. In cacti, it is the stems that are modified to store water, while in aloes, it is their leaves. A careful look at this plant reveals that it does in fact consist of a compact series of spirally arranged leaves.  

When unrelated organisms come to resemble each other, it is an example of convergent evolution - similarity due to common adaptation, but from very different ancestors.

I've spoken of this a number of times on this site, as succulents in general are a spectacular example of convergent evolution and the all-important process of adaptation. I've been retired for a number of years, but if were to go back into the biology classroom again today, I would walk up to the front and write ADAPTATION on the board (or powerpoint screen!). I would then proceed to show how everything we see in organisms is a result of this process, which unites genetics, ecology, evolution, and systematics.

Cacti and aloes have very different evolutionary histories leading to their convergence. In this post, I want to emphasize the historical dimension of adaptation. The current functional features of a plant have histories of gradual change, sometimes adding or improving functionality, sometimes changing the function altogether. Leaves themselves underwent extensive series of adaptations in early plants just to become the flat, efficient light-gathering antennae that we know today, and similar series of adaptations happened multiple times in different groups of plants. the very leafiness of leaves itself is convergent in true mosses, clubmosses, ferns, different groups of seed plants.

 In various lineages of plants, leaves were further modified into the parts of gymnosperm cones, the parts of the flower and other reproductive structures. Leaves have also been modified through adaptation into grasping tendrils, sticky insect-catching traps, and other specialized structures. In cacti, leaves disappeared, or were converted into spines, as the stems adapted simultaneously for photosynthesis and water-storage. In aloes, leaves retained their photosynthetic function, while adding water storage.

Both cacti and aloes thus came from "normal" plants adapted for less arid conditions. Among the nearest non-succulent relatives of cacti are carnations, and of aloes they are daylilies, asparagus, and amaryllis. Cacti are eudicots, which typically have prominent stem systems and relatively small leaves. Aloes are monocots, which typically have condensed, inconspicuous, and mostly underground stems, but prominent, elongate leaves. It was "easier" for cacti to adapt their already exposed stems for water storage, but for aloes it was easier to add that function to their leaves, than to redesign their underground stems.  So in adapting to new conditions, organisms modify what they already have, in the simplest way ("along the lines of least resistance").

So in considering the process of adaptation, we must keep in mind that organisms adapt to new or changing conditions by modifying pre-existing structures. Developing new organs from scratch happens rarely, if ever. In plants, stems and leaves have been the most plastic of organs, forming a wide variety of distinctive adaptive organs.

For some zoological examples, take the wings of birds. These highly specialized flight organs evolved from the front legs of their non-flying ancestors, radically changing their function.  It happened separately in flying pterosaurs and in bats, using arm and hand bones differently. It did not end there with the birds, for wings went through another transformation in penguins, from flying organs to swimming organs. In snakes,  legs disappeared altogether or remained as a set of tiny useless bones buried within their muscles, what we refer to as vestigial organs. The same thing happened in whales, where the front legs were modified into flippers, while the hind legs were reduced to buried vestigial structures. So one possible endpoint for a history of adaptations is to disappear!

Studying the history of adaptations in particular animals, or particular organs, is one of the most fascinating areas of biology, helping us understand the strange bedfellows resulting in modern classification (carnations and cacti, asparagus and aloes!), as well as the process of adaptation and the ecology of organisms. Proposed evolutionary scenarios must always include a plausible evolutionary (i.e. adaptive) history of how they came to be. 

The aloes and some closely related genera, incidentally provide another opportunity for a theme and variations expedition like I did with the Amaryllis family a little while ago. These leafy succulents are native primarily to Africa, and here are some photos from my collection:

Close-up of an unidentified Aloe
showing the emergence of new
leaves in the center.

Aloe (or Kumara) plicatilis is
unusual in having its leaves
arranged in a single plane.

Aloe (or Gonialoe) variegata

Aloe pictifolia

Aloe dichotoma is a rare example of a monocot that becomes a tree 
through an unusual type of secondary growth.

The difference between blackberries and mulberries and why it matters

Blackberries grow on prickly vines or brambles, 
and are members of the Rose Family (Rosaceae).

 As I was picking mulberries from a tree in my back yard the other day, I was reminded of the similarity between blackberries and mulberries. They are strikingly similar in appearance. 

Like most dark fruits, they are both rich in nutrients and protective phytochemicals. For the consumer, the primary differences are the somewhat milder, less sweet flavor, and the annoying little green stems of of mulberries. Depending on the climate, one or the other may be easier to grow, and fresh blackberries are generally more widely available in stores. Dried mulberries, however, are becoming increasingly popular and more available. Beyond all that, it's a matter of taste.

Mulberries grow on trees, and are 
members of the Mulberry Family
(Moraceae).

In terms of the teaching of botany and evolution, however, the similarities and differences between the two berries tell a powerful story. Though they function the same way in natural fruit dispersal, they are not related at all. Blackberries are members of the the very fruitful family, Rosaceae, which includes raspberries, strawberries, plums, cherries, peaches, apricots, apples, pears, rose hips and many more.  Mulberries, on the other hand are members of the Moraceae, which includes figs, breadfruit, and rubber trees. 

The structures of the two fruits are quite different. Blackberries are aggregate fruits, which means that the cluster of drupelets derive from a cone of separate carpels belonging to a single flower. Mulberries on the other hand are technically infructescences, or compound fruits, similar to pineapples. Each drupelet forms from its own tiny flower. So there are fundamental developmental differences that lead to the similar looking fruits.

This might seem like a geeky bit of botanical trivia that would quickly make dinner guests fall asleep, but in the classroom, however, it illustrates some of the most central phenomena of evolution: adaptation, adaptive radiation, and convergent evolution.

These fruits, first of all, are adapted for dispersal by animals, primarily birds, though I have had to keep an eye out for hungry black bears as well while picking berries along roadsides in Washington State. Both go through green and red phases before turning black at ripening. This primes the animals for the coming feast. The berries are sweet, juicy, and flavorful. The animals gobble down the fruits, and the digestive process strips away the juicy tissues, leaving the tiny seeds to pass through the alimentary canal. The animals tend to move about after feeding, leaving seeds in their feces. (See also "What is an Adaptation?)

The different kinds of fruit to be found in the Rose Family are an example of adaptive radiation - the evolution  of a variety of descendant species from a common ancestor. As the descendants of the common ancestor  began spreading into new habitats and new geographic areas, they adapted to local conditions, including local fruit dispersers. 

As other families went through their own adaptive radiations, some descendants encountered the same opportunity for dispersal, and developed similar physical characteristics, but with tell-tale differences in underlying structure. This is convergent evolution - the development of very similar adaptations from unrelated ancestors. The ancestors of the Rose Family happened to have flowers with multiple separate carpels, and so easily evolved into aggregate fruits, while the ancestors of the Mulberry Family had tiny flowers with just one carpel in each, so a similar fruit was most easily developed by grouping the fruits of many flowers together.  I have posted earlier about evolution of cactus-like members in  unrelated families, along with numerous examples of convergent evolution in animals. (See "Of cacti and humans – are certain life forms inevitable?"

Why do coconut palms lean?

 

Coconut palms commonly grow along tropical coastlines
in a zone of salt-tolerant vegetation, but not directly in
saltwater. Coconuts may fall onto the beach and be carried
away by high tides, but not usually directly into the water.

 Coconut palms have a distinctive, arching growth form, which is somewhat unusual among palms. Most solitary, tree-like palms grow straight upward rather rigidly. The reason for the coconut palm's graceful arch has led to much speculation online, some of it rather goofy, such as that they lean out over the shoreline in order to drop their coconuts into the water for dispersal. Slightly more plausible is that they lean toward the light, or that they are bent by the coastal breezes. 

While these factors may contribute somewhat to the ultimate shape of the mature palms, I'd like to point to a more fundamental factor: the phase of development that all palms go through after germination called establishment growth.  This is something peculiar to tree-like monocots, which have neither a taproot system nor layered secondary growth. In dicotyledonous trees, stem thickness increases gradually throughout the plant, and the root system branches to keep up with it. (See The Root of the Root Problem)

While coconut palms may appear to all lean toward
the ocean (to the left in this picture), they in fact may lean
inland as well, at least at the beginning. Only a few
at the far upper left of this photo are actually leaning
toward the ocean. Note that the bases of the stems emerge from
the ground at a distinct angle. This is the result of the
early phase of horizontal establishment  growth. 

Most monocots keep their main stems underground as rhizomes, corms, or bulbs, and produce adventitious roots. Leafy shoots and/or flower stalks typically arise directly from these underground stems, and die back after their reproductive cycle. Becoming trees, as in palms, screwpines (Pandanus) or traveler's "palms"  (Ravenala), was an evolutionary afterthought, for which new ways to develop trunk thickness and a sufficient root base had to be invented. (See also The Invention and Reinvention of Trees.)

Monocot trees do this by developing their full stem thickness, along with a mass of permanent adventitious roots, at, below, or close to the ground before beginning their vertical growth. The trunk base can widen only by extending more roots into the soil. This is what we call establishment growth. 

The underground stem of a cabbage
palmetto during establishment growth 
is shaped roughly like a saxophone, with 
the mouthpiece representing the seed,
and the opening of the bell
representing the ever-widening shoot
apex. You have to imagine roots
sprouting along the body of the
saxophone, and leaves emerging from the
open end of the bell. 
Drawing from Drawforkids.com.

There are several ways to do this. In cabbage palms (Sabal spp.), for example, the shoot apex first grows downward into the soil, sending up its juvenile leaves  and sprouting adventitious roots as it goes. The stem tip gradually widens and then turns upward. The overall shape of the stem at this stage resembles a saxophone. By the time the shoot apex (stem tip) reaches the soil line, it is as wide as it is going to get, and begins forming a an upright trunk. This takes some 25 years for a Sabal palm.

The production of s series of aerial stilt
roots allows this palm to increase the
thickness of its stem while growing upward.

Other palms, as well as screw pines, begin growing upward immediately out of the seed, as very slender stems that widen as they grow upwards and produce adventitious roots that remain for the life of the plant in the form of stilt roots. 

The horizontal establishment growth of the coconut
palm stem will proceed to the right in this example.
Photo by Vencel, CC attribution 3.0. 

It appears that the coconut palm follows a third model by establishing its basal thickness along with  a mass of adventitious roots, through a period of condensed horizontal growth, with the lower side of the trunk remaining in contact with the soil. Once it achieves full thickness, the trunk gradually curves upward to achieve a more-or-less upright growth, though it often continues to lean. Since a coconut seedling sprouts out of one end of the coconut, the direction of the horizontal growth phase and the eventual upward curve, will depend on which way the coconut is facing when it sprouts - not so much for any functional reason. 

This is my hypothesis anyway. Those of you who have grown coconut palms from seed can perhaps verify or correct it.