Do we owe our existence to the fig?

 Most of us hardly ever think about figs. It is one of many fruits available to us. Some could take them or leave them. It is quite possible, however, that this humble fruit played a critical role in human history, and even in the very existence of humans in the first place.  

The edible fig we know (Ficus carica)  helped sustain life for prehistoric people struggling to survive in the relatively harsh climate of the middle east. The oldest evidence of modern figs is from a site near Jericho dating 11,400 years ago. The key feature of these fossilized figs was that they were of a seedless variety that can only be propagated by cuttings. That means that figs were planted and cared for by people, making them the oldest known cultivated plant - older than wheat, rice, or any other crop. The figs found at Jericho were just the oldest to be preserved, but they were likely domesticated hundreds or thousands of years earlier, and before that even, humans (and possibly Neanderthals) most certainly consumed wild figs. 

Appropriately, the fig is a leading candidate for the "forbidden fruit" of the Garden of Eden. The fruit that tempted Adam and Eve is not named in the various versions of the creation story, and was not likely an apple as often depicted. The very first clothing of the sinful couple is identified as fig leaves, making the fig the more likely culprit. While there were wild species of apple throughout Eurasia, the tasty, sweet apple we know today was not found in the Middle East until introduced from Kazakhstan or China by traders along the Silk Road less than two thousand year ago.

So figs, which grow readily throughout the Middle East and all around the Mediterranean, produce fruit prolifically and were likely a staple of the people who would give rise to the ancient civilizations of Mesopotamia and Egypt, and may have been a life saver during lean years. 

Humans, indeed primates in general, have always been omnivores. Our ancestors hunted animals for their meat, but humans could not survive on meat alone. Carbohydrates from plants, along with certain essential nutrients like vitamin C, were necessary parts of the diet, and in this sense, figs were highly nutritious. It is also a fact that larger animals were gradually hunted to extinction in the area, and plant foods increasingly filled the gap in our energy needs. Without the fig then, the survival of human populations in the Middle East would have been more difficult, and the civilizations that arose from them, might have been delayed or conceivably stopped altogether. 

Now let's dial the prehistoric clock much further back, where figs likely played an even bigger role in our evolutionary history. The oldest known fossils of wild species of Ficus date back to the end of the Cretaceous period (66 mya). At this time, dinosaurs went extinct, and mammals began to emerge from their shadows and began to diversify. Early primates appeared at this time, characterized by an omnivorous diet and adaptations to life in rainforest canopies. Figs may have been among their food sources. Figs then appear to have undergone a major radiation around 20-40 million years ago, spreading to tropical forests around the world. Coincidentally, this is the time when true monkeys evolved from earlier primate ancestors and also spread around the tropical world. There are now at least 800 species of Ficus, all consumed by animals who spread their seeds in their feces. 

The grasping hands of monkeys and other
primates are important both for grasping
slender branches and for manipulating
food items. 
Photo by Whaldener Endo CC BY 2.5

Monkeys perfected life in the trees, developing great mobility and dexterity in the pursuit of their omnivorous diet, which included fruits, seeds, leafy shoots, flower parts and, to a lesser extent, small animals (insects, lizards, frogs, and bird eggs). It is an active life style, requiring continuous movement through the forest, and an energy-rich diet. Though omnivores, monkeys rely primarily on the carbohydrates of fruits for their energy needs. To find, pick, and manipulate such foods, monkeys have to move efficiently through the interlaced branches and vines of the tropical forest, from one fruiting tree to another. Their extraordinary agility and body flexibility would later be repurposed for developing technology needed by early humans for life in the savannas. Their specific adaptations include grasping hands, opposable thumbs, sensitive finger tips with flat nails, rotatable shoulder and arm joints, binocular color vision, intelligence, communication, and social interaction. (See also Of cacti and humans - are some species inevitable? and The problem with Wookies.) 

The proliferation of fig species in tropical rainforests may have provided the abundant and steady supply of carbohydrates that made the monkey lifestyle possible. Why figs and not the other tropical fruits?

The inside-out receptacle of a fig,
bearing many tiny flowers on its
internal surface. from Lessons
with plants by Liberty Hyde
Bailey, 1898

It's time to look at the unique structure and biology of figs. A fig is not a simple fruit, but rather a compound fruit, like a pineapple or mulberry, derived from multiple tiny, crowded flowers. It is actually a many-flowered inflorescence. In the mulberry, which is in the same family (Moraceae), the small flowers are borne along a "normal" elongate stalk (receptacle). In a fig, however, the receptacle expands outward and upward, enclosing the tiny flowers in a chamber called a synconium. 

The flattened inflorescence of Dorstenia. Such
flowers are pollinated by various small insects,
including wasps, and maybe by the wind.
Photo by Nyanatusita - Own work, CC BY-SA 4.0

To understand this we can look at another relative of the fig. In Dorstenia the receptacle flattens out into a plate-like structure with the many small flowers borne on its surface. This might be considered an "almost fig." If its edges were to curve upwards and close at the tip, it would have the basic structure of the fig.

This unique floral structure is an adaptation for pollination by highly-specialized tiny wasps. it is believed that each of the 800 or so species of Ficus has its own species of pollinating wasp. 

There are variations among the 800 species of fig wasp, but a typical life cycle is as follows: A female wasp enters the fig through a tiny opening at the tip, and inserts a pre-fertilized egg inside a female flower where it develops into a larva. The larva develops by feeding on the tissues of the fig, and emerges inside the synconium as either a male or a female. This may take several weeks to several months, depending on the particular species of fig. The male wasps never leave the fig, but fertilize emerging females, often even before they are fully mature. The males then die and their tissues absorbed by the plant. The female wasps mature into their adult winged form and then leave the fig. At that time, the male flowers of the fig mature, and dust the female wasps with pollen as they leave. 

The females do not feed and live only a matter of days outside of the fig. They seek out and enter another young fig, where the reproductive cycle begins anew. The pollen on their bodies brushes off on  female flowers in the new fig. After they lay their eggs, the female wasps die and their bodies are absorbed. So the longest part of the fig wasp's life, by far, is within the fig.

This is a mutual, obligatory relationship between wild figs and their wasps. Without the wasps, the figs cannot be pollinated and reproduce. Without the figs, the wasps cannot survive and reproduce. Since the female wasps cannot survive outside of the fig for very long, there must be newly forming figs constantly available. So tropical fig species continuously bear fruit. There are questions about how fig wasps overwinter in temperate species, most likely in partially developed dormant figs, but this is not relevant to the current story.

The bottom line is that tropical fig trees produce fruit continuously, providing a reliable, abundant food source for many rain forest inhabitants, including monkeys. It is quite possible then, that without figs, there may not have been sufficient reliable food for monkeys to have evolved, diversified, and developed their unique physical dexterity.

Tropical rain forest trees with more generalized pollinators, such as bees, butterflies, birds, bats, and beetles, may be seasonal in fruit production, with periods focused on vegetative growth or  surviving periods of reduced rainfall. So at times, figs may be the only food available in sufficient quantity to support fruit-eating animals. Monkeys also eat some leafy shoots, insects, etc. but this may not be sufficient to sustain a population for the long-term. 

Primates and figs came together just at the right time and place to set human evolution into motion. Could other arboreal mammals have done it?  Sloths, koala bears, and squirrels started off differently. These animals based their tree-climbing ability on claws, rather than the unique grasping hands and sensitive fingers of primates. Sloths and koala bears are primarily leaf-eaters, and so are low-energy arborealists. They could not compete with monkeys. Squirrels evolved in temperate regions where going up and down tree trunks with the aid of claws was more important than swinging through the tree-tops. Though they are somewhat omnivorous and quite agile, their diet, and primary carbohydrate source, center around seasonal hard nuts, which they bury for use in the winter. Claws are thus also required for digging. These adaptations were set in stone, even before some squirrels moved into tropical forests. 

Anyway, that is my hypothesis: without figs 20 million years ago, no monkeys - no monkeys, no humans today.

 

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?