Pitfalls of the long branch

Long branches in phylogenetic trees represent lineages of  organisms that have been around for a long time, but exist today as only one or a few species.  A few years ago, I discussed two examples in detail: the monocot genus, Acorus, and the archaic angiosperm species Amborella trichopoda.  I feel that the topic is worthy of a review, especially for newer readers who may not have gone back to the older posts. 

In both cases, these lineages branched off very early, over 100 million years ago, but have left no fossils, and have no close living relatives.  The Amborella branch is the earliest surviving lineage of angiosperms in general, while the Acorus branch is the earliest surviving lineage of monocots.  Expressed in a different way, Amborella is the sister group to all other angiosperms, and Acorus is the sister group to all other monocots.

At the level of phylogenetic analysis, such long branches have often been problematical, with "long branch attraction" leading occasionally to errors in the resulting phylogenetic tree.  This has been much discussed, and there are ways to correct for it, but this is a very technical issue. If you want to learn more, you might begin with .  Begin with this Wikipedia article, and go from there.

 In both cases, however, many phylogenetic analyses have confirmed the ancient position, and length of these two branches, so that is not a question  here..

Such long branches can lead to errors of interpretation at another level, however. A common misconception is that what we see in the current species, which occupy the very tips of these ancient lineages, will be similar to the  ancestors from which the lineage began, i.e. that these are archaic or primitive species.

But think about it.  These lineages have been around for more than 100 million years  (140 million for Amborella, 120 million  for Acorus).  Isn't it likely that the occupants of these lineages have changed somewhat over all those years?

Amborella fruits are single-seeded drupes, adapted for
dispersal by fruit-eating birds.  This is a specialization
that has evolved many times among angiosperms, including
most famously, cherries. Early angiosperms most
likely had fruits that split open to release several to many
seeds (see Were the first carpels plicate or ascidiate? . 
Small, unisexual flowers in dense clusters
 is also a specialization. Photo courtesy Joel McNeal.

Modern phylogenetic analyses are based primarily on molecular (DNA) comparisons, so in-and-of themselves tell us nothing about changes in the characteristics of the plants occupying the lineages.  So there is no direct basis for inferring what the first species in a lineage looked like or in  what ways their modern descendants may have changed.

As I argued in the previous posts, both Acorus and Amborella, as they exist today, exhibit a mix of ancient and specialized characteristics. They are both well-adapted to their environments, and have some distinctive specialized characteristics, particularly in their adaptations for pollination and seed dispersal. The Acorus and Amborella lineages have been around for such a long time, that it is rather absurd to think that they have not changed at all during that time. For groups that have good fossil records, we can trace such changes.  Fossils, for example, tell us that we modern humans have changed a great deal from the first members of our genus, even more from the ancestral genus Australopithecus!

The spadix-like inflorescence of Acorus led early
taxonomists to classify this genus with the Aroids.
Since the two families are not closely related, it is likely 
that the similarity is due to convergent evolution, driven by
adaptations for pollination. A spadix is a highly specialized
 way to arrange flowers and has evolved independently in a
number of families, including the Aroid, Palm, and 
Cyclanthus families. It is likely that the early monocots had
looser arrangements of flowers, more like those in most
Alismatales, and that dense flower spikes were not
characteristic of the first members of the lineage.
The folded and fused (equitant) leaves of Acorus, are
also a specialized adaptation that has occurred in many
unrelated families, most famously in several members
of the Iris Family.

How do we know, or at least develop hypotheses, as to what changes have taken place in a lineage in the absence of any fossils?  We can look at the characteristics of other early branches to see what they have in common, and hypothesize that the shared characteristics were present in their common ancestor.

 We can also analyze how particular characteristics might have arisen as adaptations to natural selective pressures, and determine which are most likely ancestral, and which are more specialized. Adaptations arise in logical sequences and often become canalized in non-reversible directions (see What is an adaptation? and G. L. Stebbins and the process of adaptive modification)

Both in comparison with other related groups, and in considering likely sequences of adaptations, Amborella and Acorus are specialized in some ways. For Amborella, small, numerous, unisexual flowers in clusters, and red, single-seeded fruits are both features that are more specialized than in other archaic angiosperms. For Acorus, the dense spikes of flowers with fused carpels (see also Were the first moncots syncarpous?) and the the leaves with the two sides fused together (equitqnt are specialized features, that have evolved independently in a number of families from more generalized types.

Guide to the mosses of central Florida

I have spent much time in the past few years studying the mosses of central Florida and posting portraits of the common species.  This work has culminated in a Guide and Interactive key, which has now been posted as part of the Atlas of Florida Plants..You can find the link on the Atlas home page, in the right column.

The guide is in  pdf format, so no special software is needed to use the key.  Though it can be printed

Page 1 of the key, showing the three initial choices. Move
through the key by clicking on arrows. One can also go
directly to the index by clicking the box in the upper right.

out, it is designed to be used on-line, or off-line with computer or cell phone, as there are active links leading the user from one part of the guide to another.

The key leads to species profile pages, similar to the postings I have done on this blog site, but briefer.  Currently, the guide covers 59 common mosses of central Florida, but as stated in the introduction, it is a work in progress.  The index at the back of the guide includes all species reported as occurring in Florida, and as we are able to include additional species, the guide potentially will morph into one that covers the entire state.  The guide will therefore be periodically upgraded and reposted. Those of you who have been following the moss posts are encouraged to notify me of any errors you see in the key, any information or photos of additional species, or any other suggestions.

Thanks for your interest and support.

On the species profile pages, one can find photographs, maps of distribution, and a link to the species page in the Atlas (logo under the map).  One can also go to the  index to see what other species are reported from Florida, or return to the key.

Theme and Variation - the Amaryllidaceae

The primary types of cultivated amaryllis are in the genus Hippeastrum.  

Their flowers are mostly shades and mixes of red, pink, and white. 

This is  one of my favorite cultivars, "Eye-catcher."

This spring, while I was waiting eagerly for the amaryllis plants in my yard to bloom, I started reflecting on
the marvelous family to which they belong, and how nicely they represent a fascinating aspect of plant evolution.


The Amaryllis family is known and beloved worldwide, even by people unfamiliar with its technical name or taxonomy, for it provides us with a variety of unique spring-flowering bulbs and perennials, from daffodils to subtropical amaryllis and tropical Crinums.

As presently defined, Amaryllis (technically the genus Hippeastrum), daffodils (Narcissus) and Crinum all belong to the subfamily Amarylliodeae. Onion, garlic, etc.are also members of the family, constituting the subfamily Allioideae.   Finally, the blue-flowered "Lily-of-the-Nile" (Agapanthus), from southern Africa, is technically in it's own subfamily, Agapanthoideae.  Altogether, there are some 1600 species in 75 genera, found naturally on every unfrozen continent.

Daffodils are specialized members of  the genus Narcissus, in which the umbel has been reduced to a single flower.

The subfamily Agapanthoideae consists of the single genus Agapanthus from
southern Africa. Flowers are blue to white.

The onion subfamily,  Allioideae, contains numerous aromatic and edible species.
The characteristic pungent fragrances are based on allyl sulfides,
which in nature  act as deterrents to insect pests.

The true bulbs of onions and amaryllis are made up of
the swollen bases of recent leaves that encircle the
central stem.  The outermost layers, representing
 older leaf bases, become dried and paper-like,
 which protects the fresh inner layers from drying out.
 This gives rise to the designation "tunicate bulbs,"
differing from the scale-bulbs of the true lilies.
Photo by Amada44, CC BY-SA 3.0.

So what defines this family? What is the common theme upon which the 1600 species are variants? The vast majority of the species in this family are geophytes, plants that survive adverse seasons underground. Most species form bulbs, but some, like Agapanthus and certain members of the Allioideae, employ underground rhizomes instead.  The leaves are strap-shaped (sometimes tubular and hollow in the onions) and extend themselves upward from the bulb by basal intercalary meristems (see "How the grass leaf got its stripes").  This is the most common form of leaf in the monocots, and it varies little in this family.

True, or tunicate bulbs (see illustration to the left), differing from the scale-bulbs of the true lilies, do seem to be a unique invention of this family,  though some members of the Lily family, such as tulips, have evolved a similar type of bulb independently.

Flowers in the Amaryllidaceae  undergo preliminary 
development below ground, between leaves or within the 
bulb and are protected by a closed sheath.  The enclosed 
bud is then pushed upward by the intercalary growth of  
the stalk.

But it is how the flowers emerge from the bulbs that is the most iconic, revolutionary, and consistent theme of the family.  Flowers form below ground, tightly enveloped in a protective sheath.   Below each inflorescence bud, a stalk (the peduncle) develops and lengthens through basal intercalary growth (i.e. new tissues are produced at the base of the stalk, pushing older tissues and the inflorescence bud upwards).  After rising to optimum height for pollination and eventual seed dispersal, the sheath splits open to reveal a simple umbel, i.e. one to many flowers arising from a single point at the tip of the stalk, roughly forming the shape of an umbrella or sometimes an entire sphere.

This proved to be a remarkably effective way to protect and elevate the flowers, for after it evolved in the common ancestors of the family, descendant species spread worldwide, adapting to different climates, soils, and pollinators. Such a spreading diversification is called an adaptive radiation. Note that the special structure and growth form of the inflorescence remained essentially unchanged throughout the family, while details of flower structure and color, fruit type, and physiological adaptations diversified.

Yellow flowers are uncommon in the Amaryllidaceae, but found here in
Lycoris aureus.  Photo by Tomago Moffle, CC BY-SA 3.0.

 The importance of this discussion is not simply to say how wonderful and unique the Amaryllidaceae is, but to stimulate us, particularly those of us who are teachers, to look for similar patterns of breakthrough adaptations followed by adaptive radiation throughout the plant kingdom.

Almost any genus, and sometimes a whole family can be seen to be based on some "great idea," i.e. some new structure, growth pattern, flower type, etc., that gave the ancestral species an advantage and allowed its descendants to diversify into great numbers.  Two simple examples are the genus Aquilegia (columbines) with its nectar spurs arising from each of the five petals, and the genus Euphorbia, with its highly compact flowering units called cyathia.

How many examples can you find? Can you explain the adaptive value of the distinctive features?

Each yellowish, red-tipped structure in this Poinsettia
(genus Euphorbia) is a cyathium, a cupule containing several
tiny flowers.

The highly distinctive flowers of Aquilegia feature a nectar
spur projecting backwards from each petal.

The giant crinum, C. asiaticum, from southern China, is a tropical evergreen
plant that develops a pseudostem, similar to that of the banana, made of the
tubular bases of the leaves.

Mosses of Central Florida 52. Gemmabryum apiculatum

Gemmabrum apiculatum forms thick cushions, with well-spaced leaves on
the shoots.

Gemmabryum apiculatum (Schwagrichen) J. R. Spence & H. P. Ramsay (Bryaceae) forms colonies of tiny, upright leafy shoots on damp soil in shady areas.  Leaves are long-ovate, well-spaced along the stems, and mostly 1 mm or more in length. Leaf cells are narrower than in related species, 6 to 8 times longer than wide, and become square toward the base.

The species characteristically forms tiny reproductive tubers or bulbils along the rhizoids in the soil or in the axils of the leaves. Bulbils are brown, pear-shaped, and 40-80 µm long. I have not yet seen spore capsules in our area.

Brown, pear-shaped bulbils in the leaf axils are characteristic of
Gemmabryum apiculatum. Photo by Ainun Nadhifah

Gemmabryum apiculatum is probably to be found throughout the state, as it is found  in coastal regions of other southeastern states, though our documented specimens are from central Florida southward. It is also found widely in the tropics. 
G. coronatum has a similar distribution, with some reports from the north.  The leaves tend to be rolled at the margins, and the leave cells are shorter, 3-4 times as long as wide.
A third species, G. exile, has been reported only from Collier County, but is easily recognized by its stringy stems and small, folded leaves.

The leaf of Gemmabryum apiculatum features a strong
 midrib, and elongate cells that become squarish toward the base,
Photo by Ainun Nadhifah

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Mosses of Central Florida 15. Physcomitrium pyriforme

Physcomitrium pyriforme forms extensive colonies, and an abundance

of spore capsules, in the wet soil along receding ponds. (Essig 20160328-1, USF)

[Note: this species was previously posted incorrectly as Physcomitrium collenchymatum]
[For other mosses in this series, see the Table of Contents]

Physcomitrium pyriforme (Funariaceae) occurs along the receding edges of ponds during the dry season, and in other disturbed wet sites.  It evidently completes its life cycle rapidly, producing an abundance of spore-bearing capsules in the interval before the rains fill up the ponds again.

After losing their lids (calyptras) the capsules resemble
 wide-mouthed wine glasses and lack teeth around the margins.  

This species occurs in Florida and in other southeastern states, with outlying records in Kansas and Nova Scotia.  It is distinguished from the related species, P. collenchymatum, by its  inverted pear-shaped,  rather than globose, capsules.  The capsules lack any teeth around the opening, which distinguishes them from many common mosses, such as Isopterygium.

The leaves have a strong midrib and clear, rectangular to angular cells with walls irregularly thickened.  The thickened appearance appears to be due to chloroplasts adhering to the walls.  Leaf cells are smooth, lacking any papillae (hard, pimple-like bumps).  This distinguishes this species from similar-looking members of the Pottiaceae.

The leaves of Physcomitrium have a strong midrib, and large rectangular cells.

Note: photographs, geographic distributions and information about the naming history and synonyms of this and other mosses are currently being incorporated into the Atlas of Florida Plants.

Adhering chloroplasts give the cell walls a rough, thickened
appearance.

Cyanobacteria - superheroes of evolution

The first plants, photosynthetic cyanobacteria, are still
abundant in a great variety of forms today and account for
about 20-30% of the current oxygen production in the
oceans.  photo by Mary Cousins cc by SA 3.0

Though it's not obvious from the name, cyanobacteria are photosynthetic organisms.  They were actually called "blue-green algae" until it became evident that they are prokaryotes related to other bacteria.  They thus have a simpler cell structure than the "true" eukaryotic algae and higher plants.

The ancestors of modern cyanobacteria invented photosynthesis over 3 billion years ago, or I should say they assembled it from processes obtained through horizontal gene transfer from at least three different ancient bacteria.  (See "The first plants") Earlier bacteria that harvested sunlight created carbohydrate, but the cyanobacterial process also releases releases oxygen as a byproduct, and therein lies the most important part of the story.

Cyanobacteria, such as this filamentous Oscillatoria were
classified as algae until their prokaryotic nature was discovered.
Photo by Wiedehopf20. CC SA 4.0 International

Chroococcus is a cyanobacterium in which
cells divide within a gelatinous  matrix. 
Photo by Xvazquez CC by 3.0 unported.

For the first 2 billion years of their existence, cyanobacteria alone served as the base of the world's food chain, providing vast quantities of carbohydrate to feed the rest of life.  That in itself was a stupendous contribution by this group of organisms, but it was the production of oxygen that changed the world forever, making more complex plants, fungi, and animals not only possible, but obligatory and inevitable. By producing oxygen as a byproduct, cyanobacteria converted the primordially anaerobic world into an aerobic one, which both enabled and forced the evolution of a variety of organisms that could note only tolerate the toxic effects of oxygen, but also put it to use through aerobic respiration. At the same time, countless and largely unknown anaerobic organisms became extinct. It may have been the first mass extinction of life on Earth.  That, however, didn't happen right away - there was a 2 billion year delay - for a simple reason we'll see shortly.

All protists, animals, fungi, and higher plants are composed of eukaryotic cells.  Bacteria and archaeans have the much smaller and structurally simpler cell structure we call prokaryotic. Though the name “eukaryote” emphasizes the presence of a true nucleus, these more advanced cells have additional complex organelles, internal membrane systems, and a sophisticated cytoskeleton that controls cell shape, cell and nuclear division, and the movement of organelles and materials within the cell.  Such complex cells require a lot of energy for all this internal activity, and so could not have existed until oxygen was available.

When oxygen did finally become abundant in the seas, aerobic bacteria evolved, first to protect themselves, and then to harness the oxidative power of oxygen to break food materials more completely. What a boon that was!  Anaerobic respiration can squeeze only 2 ATP molecules from a molecule of glucose, while aerobic respiration yields 38!

The first eukaryotic cell evolved as a flexible, amoeba-like archaean
ingested aerobic bacteria, which evolved into  mitochondria. Later,
cyanobacteria were captured by a primitive eukaryote, and became the
chloroplasts of the first true algae.

Once aerobic bacteria evolved, another kind of ancient prokaryote took a shortcut to adapt to the oxygen-rich world.  This one, an
archaean, had already shed the rigid cell wall that surrounds most prokaryotes, and with its naked, flexible wall it could surround other cells and bring them inside for digestion.  It was essentially a primitive amoeba with a rudimentary cytoskeleton.
Such a cell eventually met up with an aerobic bacterium, related to modern purple non-sulfur bacteria, and took it inside.  Instead of digesting the bacterium, however, a truce developed between the two cells and a deal was struck.  The host cell provided food to the captured aerobic bacterium, and the bacterium in turn absorbed oxygen, used it to break down the food, and paid rent to the host cell in the form of ATP molecules.  That captured aerobic bacterium evolved into the energy–processing organelle we call the mitochondrion, and that symbiotic union was the first eukaryotic cell.  Incidentally, but also of huge significance, cyanobacteria were also engulfed by some early eukaryotes and became the chloroplasts we find in algae and higher plants.

This formation of the first eukaryotic cell has been considered by some scientists to have been particularly unlikely, and that only by chance did life on Earth therefore progress from the prokaryotic to the eukaryotic level of complexity.  In his Scientific American article of September 2018 ("Alone in the Galaxy"), John Gribbin says “it is a measure of how unlikely such a single fusion of cells was that it took two billion years of evolution to occur.” (italics added for emphasis.)  This implies that the evolutionary progress of life was stalled for two billion years and may never have gone on to form eukaryotic cells (and eventually humans), if this rare fluke of an event hadn’t happened.  Gribbin and others (including the late Stephen J. Gould) believe humans exist in the universe only because of a series of such flukes. (see references below).

As I proposed in my last post (Of cacti and humans – are certain life forms inevitable?) the evolution of life did not proceed through miraculous flukes, but rather inevitably through predictable processes. In the first place, symbiosis among prokaryotes is exceedingly common.  In fact, only two months after Gribbin’s article, another article showed up in Scientific America, which suggested that symbiotic cooperation among prokaryotes might be the rule rather than the exception (see “Team Players,” by Jeffrey Marlow and Rogier Braakman, Scientific American, November, 2018). In addition, “The Runes of Evolution, by Simon Conway Morris (2015), provides abundant examples of ways in which symbiosis occurs among microorganisms, as well as providing a veritable encyclopedia of convergent evolution throughout the kingdoms of life. So it is really quite predictable that a flexible, carnivorous archaean would sooner or later run into an aerobic bacterium, ingest it, and eventually domesticate it into an internal organelle that would help it extract more energy from food items. 

So what was the real reason for the 2 billion year delay?  Iron. At the beginning, there was a huge amount of dissolved iron in the oceans as well as in the surface rocks.   When exposed to oxygen, iron rusts, as anyone who has left tools outside too long knows. Technically speaking, this early iron was in its reduced state, and when exposed to oxygen it became oxidized. The oxidized form of iron, however, is not soluble in water, so it settled out, creating vast sedimentary deposits known as the banded iron formations. So all the oxygen produced by cyanobacteria was at first gobbled up by the huge amount of iron dissolved in the seas. Only after most of the dissolved iron in the oceans was depleted could oxygen start to accumulate in the environment, and that is what took 2 billion years - exactly the amount of time that some scientists propose that life was waiting for a fluke event to occur. On the contrary, it seems that the origin of eukaryotes happened as soon as it became possible.

So cyanobacteria were the first to feed the world through modern photosynthesis, they created a crisis that enabled and forced the evolution of aerobic bacteria and the first eukaryotes, and they became the chloroplasts for all eukaryotic algae and plants.  Anything else? Actually, the cyanobacteria probably also invented aerobic respiration itself and passed it on to other bacteria through horizontal gene transfer.

Cyanobacteria had to have a means for protecting themselves from the oxygen they produced, and also for burning the fuel they created through photosynthesis.  They most likely did this at first by running parts of the photosynthetic process backwards.  Look at schematic diagrams of photosynthesis and aerobic respiration.  Though details have changed over time, the two processes are roughly mirror images of each other.  In modern cyanobacteria, photosynthesis and aerobic respiration take place in separate pathways, but these overlap and use some of the same protein complexes.(See Photosynthesis and Respiration in Cyanobacteria, by W. Vermaas)

Stromatolites, like these in Shark Bay, Western Australia, are rare today, but
were abundant in the past.  They are constructed as mats of cyanobacteria
and other microorganisms are laid down one on top of another.  Such
oxygen-rich microenvironments may have been where the first aerobic
bacteria evolved.  Photo by Paul Harrison, posted in
Wikipedia, CC BY-SA 3.0.

The evolution of aerobic bacteria  might have started somewhat earlier than the appearance of oxygen in the open oceans.  Many cyanobacteria lived in peculiar formations called stromatolites, some of which are still around today.  These knobby pillars are formed as cyanobacteria and other microorganisms build sticky mats on their upper surface.  Within these mats, oxygen may have built up locally, creating a microenvironment in which aerobic bacteria may have first evolved. There would be little fossil evidence of such soft-bodied eukaryotes, even after they moved into the open seas.  Our first real evidence of eukaryotes in the fossil record was of algae who had durable cell walls. (see my earlier post on endosymbiosis.)

If all that is not enough to call cyanobacteria superheroes of evolution, there is one more thing.  Whether they invented the process themselves or acquired it from some ancient bacterium through lateral gene transfer, cyanobacteria were and still are major fixers of nitrogen.  This all-important process converts atmospheric nitrogen, which is inert, into ammonia, which organisms can use to make proteins, nucleotides, and countless other essential molecules.  Kudos to the cyanobacteria.

BTW - Much of this is discussed in more detail in my book, Plant Life (if you haven’t read my book yet, why not?!)

References:

Gould, Stephen Jay. 1989. Wonderful Life: The Burgess Shale and the Nature of History. Norton &^ Co

Gribbin, John. 2011. Alone in the Universe - Why our planet is unique. Wiley. NY.

Ward, Peter D and Donald Brownlee. 2000. Rare Earth - Why complex life is uncommon in the universe. Copernicus/ Springer-Verlag. NY.

Conway Morris, Simon. 2015. The runes of evolution: how the universe became self-aware.  Templeton Press. West Conshohoken, PA.

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Of cacti and humans – are certain life forms inevitable?

The search for alien life has been going on for almost 60 years, but so far, no one in the galaxy has returned our call. Why?  There are three commonly cited reasons:

1. Earth-like planets are very rare. Such planets have to be the right size, in the “Goldilocks zone” of their solar system (not too hot, not too cold, with abundant liquid water on the surface), the right distance from the chaotic center of the galaxy, have a metallic core, a relatively thin, mobile crust, and a protective magnetic field.  Nevertheless, with at least 100 billion stars in our galaxy there should be a few such planets around.  

2. The likelihood that alien technological civilizations occurring at the same time as ours is small. Planetary systems are of different ages within our galaxy.  Neither a paleolithic culture nor a long dead alien civilization, let alone a primeval world inhabited only by bacteria, is going to send or receive any radio signals.  In the 3.5 billion years life has been on our planet, we ourselves have only been capable of such communication for about 100 years.  Finding Paleolithic cultures (like the fictional Na'vi in Avatar) or the ruins of an ancient culture would be almost as much fun, but it's going to take more work than looking for radio signals.

3.  Technological humans evolved on Earth as a result of a series of lucky accidents that are unlikely to ever occur again on another planet.  John Gribbin, for example, in the September 2018 issue of Scientific American, contends that “Perhaps the most unlikely of all was the development of our technological species from those first sparks of life – a feat that is probably unique.”  This particular argument is one, however, that I believe is flawed and overly pessimistic. 

The question of alien life is analogous to asking “are there cactus-like plants in Africa?” That may seem like a stretch, but bear with me. In any case, it’s a good excuse to talk about aliens in a blog site devoted to plants! It is also a valuable opportunity to talk about the process of evolution.

It is true that Homo sapiens exists today as the result of a long line of very specific events.  My argument, however, is that similar results can arise in alternate timelines (or other planets) involving different lineages of organisms. This is because of the power of natural selection to create, often repeatedly, distinctive life forms that play particular roles (i.e. fill particular niches) in the ecosystem. The often startling evidence of this power is the phenomenon of convergent evolution - the evolution of similar organisms from unrelated ancestors. In other words, life on Earth-like planets will progress in predictable ways, becoming more diverse and complex, and result in a similar mix of life forms. 

So, are there cactus-like plants in Africa?  We have the advantage of being able to go to Africa and have a look - something we can't yet do on other planets.  The cactus family evolved in the New World and did not spread into Africa until very recent times.  However, two unrelated families of plants contain numerous remarkably cactus-like plants: the Asclepiadaceae (Milkweed Family) and the Euphorbiaceae (family of spurges, poinsettias and rubber trees).  The remarkable similarity among these plants is a spectacular example of convergent evolution. I might be so bold as to say that, given the existence of a variety of angiosperms to start with, the evolution of cacti and cactus-like plants was inevitable, because of their special ability to survive in desert environments.  Further, if all cactus-like plants were to become extinct for some reason, they would probably re-evolve from one plant family or another.  

The evolution of cactus-like plants is driven by an ecological opportunity - the existence of abundant sunlight in an area where the lack of water prevents most plants from growing. Evolving from forest shrubs, the ancestors of cacti gradually honed their anatomy and physiology through natural selection, becoming adapted to survive in the deserts. The distinctive leafless, swollen, water-filled stems of cacti are what we call stem succulents, and we can say that they occupy a very distinctive niche in the desert environment of the New World.  

Underutilized light, incidentally, was the ecological opportunity that drove aquatic algae to adapt to life in the terrestrial environment, becoming modern plants. Before that even, vast untapped sunlight is what drove the evolution of photosynthesis itself among ancient bacteria. Nature, via natural selection, does not let any resource go to waste indefinitely.

Stem succulents in the Cactaceae, Euphorbiaceae, and Asclepiadaceae have remarkably similar body forms due to convergent evolution.  Pictured are Mammillaria dioica from California (left), Euphorbia stellospina (center) from South Africa, and Larryleachia cactiformis (right), also from South Africa.

I could give dozens of other examples of convergent evolution in plants, including carnivorous pitcher plants, hummingbird pollinated flowers, and even simple categories such as "tree," "vine," or "geophyte," each of which has evolved many times in different plant families.  My argument will be that what is true for cactus and cactus-like plants on different continents could be true for humans and human-like species on different planets.

These remarkably dolphin-like animals are all reptiles

that existed in the Mesozoic era. Illustration posted on

Wikipedia, Creative Commons license.

Has convergent evolution occurred also in animals? Yes! Examples abound, including famously the repeated evolution of porpoise-like animals.   Among the Mesozoic reptiles known as Ichthyosaurs, many porpoise-like species, whose ancestors first lived on land, were hunting fish in the sea.  They themselves were repeating the body form of earlier bony fish and sharks.  True porpoises and whales were later iterations from land-dwelling mammals, but of course with more advanced mammalian anatomy and physiology. Sea lions, manatees, and penguins independently adopted the same basic body form that allows for efficient movement underwater.  Surely life on other planets will include one or more such fish-shaped animals.

Convergent evolution has resulted in numerous parallel life

forms between the marsupials of Australia and the placental

mammals of Africa and the northern continents.  Image

posted online at BiologyWriter.

Perhaps the most famous example of convergent evolution among mammals is between marsupials in Australia and placentals in the rest of the world. Marsupials entered Australia early on, while placentals either did not, or for some reason died out.  So as placentals adapted to the varied habitats and food resources in Africa and the northern continents, marsupials adapted to similar habitats and food sources in Australia.  So in Australia there are marsupial wolves, mice, cats, moles, anteaters, and grazers.  There are even arboreal marsupials called cuscuses that are like  primitive primates.  It seems then that these distinctive life forms appear inevitably wherever there are similar habitats and food sources.  In other words, when there is an ecological opportunity (an “empty niche,” if you like), organisms will adapt to exploit it.  

So if porpoise-like, wolf-like, mouse-like, and other animal life forms are evolve inevitably in response to particular ecological opportunities, why not humans? What does it take to make a human?  

To begin with, humans are mammals, rather than reptiles or birds.  Live birth, maternal feeding through lactation, and the accompanying parental care seem to foster the ability to learn or pass information among individuals, an important component of human behavior and intelligence. The ancestors of primates were probably small, shrew-like mammals that had co-existed with dinosaurs for millions of years, primarily by staying out of their way, coming out at night, or by scampering up into trees.    

Figs arose about 50 million years ago, during the early
diversification of primates, and are a major source of food
for birds, primates, and other animals in tropical forests
around the world..
Photo: Bernard DUPONT, Flickr CC BY-SA 2.0

Primates are traditionally thought to have appeared around 55 million years ago, when they began showing up in the fossil record.  A radically new theory, however, places their origin at about 185 million years ago, with other estimates inbetween. Either way, their diversification in the fossil record coincides with the development of tropical rain forests increasingly dominated by angiosperms, particularly those producing tasty fruits.  Tropical figs (of which there are some 850 species occurring throughout the tropics) first appear in the fossil record about 50 million years ago and have been a major food source for tree-dwelling primates, as well as birds and other arboreal animals ever since.  Angiosperm trees also have broad crowns with intermingling limbs, and some angiosperms evolved also into lianas that further lace the trees together.  This created a 3-dimensional, food-filled jungle gym perfect for the evolution of primates - an ecological opportunity that would almost certainly be exploited in any timeline or planet. 

Primates have flexible shoulder joints allowing for rotatable 
limbs, as well as grasping hands and stereoscopic, 
color vision. Photo posted by Glen Tarr on Quora.com.

Humans could not have evolved without some key anatomical innovations that arose during our ancestors' arboreal phase.  Yes, our ancestors had to have been monkey-like creatures, not dogs or gazelles or dolphins or octopi, and here’s why.  The hallmark feature of humanity is our ability to fashion weapons, tools, clothing, musical instruments, and ultimately space ships out of sticks, stones, and other natural materials.

To do all of that we need grasping hands with opposable thumbs, eyes forward on relatively flat faces for stereoscopic vision, and something that doesn’t get as much press coverage: rotatable limbs -  arms that can rotate around the shoulder joint and be lifted above the head. Imagine a dog or a gazelle walking up to a pitcher’s mound, winding up, and throwing a fastball across home plate.  They can’t do it because of the limited flexibility of their shoulder joints. Rotatable limbs evolved in primates, along with grasping hands, as a means of swinging from branch to branch in a tropical forest, as well as for reaching edible fruits and other food items in those trees. Such limbs were essential for the earliest stages of weapon technology - throwing rocks and wielding sticks.

The second crucial phase of human evolution, was, ironically, coming back out of the forest, but now equipped with grasping hands, rotatable limbs, and a higher level of intelligence, communication, and social interaction that also progressed in the trees.  Anthropologists still debate the ecological stimulus for the human debut, but it appears that a changing climate in Africa led to expansion of the savannas and greater variation of available food sources.  The evolving intelligence of our ancestors enabled them to recognize patterns and predict where to find food in this sparse habitat, as well as to devise cooperative strategies and weapons for hunting and defending themselves.  In the process, they became more upright in posture, perhaps to be able to see over the tall grass, but also to more comfortably carry and use weapons.  

Incidentally, when I was teaching introductory biology I did an exercise with my students critiquing fictional aliens.  It is highly likely that technological, human-like aliens would have had to go through a similar evolutionary sequence as us, and therefore would look boringly like us.  They too would first have to have lived in tropical trees and then have come down from those trees to have the necessary hand and shoulder anatomy for manipulating weapons.  So which are evolutionarily more logical - Hutts or Vulcans? Try to imagine how each would have evolved their human-like characteristics in some other way.

As intelligent as octopi and dolphins might be, they could progress no further in the aquatic environment.  Though octopi have some ability to manipulate objects with their tentacles, dolphins have none. Aside from that, have you  ever tried to throw a rock underwater?  How about lighting a fire or smelting iron for weaponry or tools?  Forget also Lady Proxima, the giant, apparently amphibious, worm-like creature portrayed in the most recent Star Wars Film, Solo - how could such a creature evolve into an intelligent being?

Was the evolution of human-like creatures  inevitable and repeatable?  All we can say is that it was ultimately  successful (perhaps too successful given our precarious environmental situation today!) and that under similar circumstances on another planet it would probably happen again.  If tropical forests filled with edible fruits evolve, creatures will adapt to arboreal life.  Climate change will inevitably happen, and primate-like creatures will very likely emerge from the forests and survive by their wits and their weapons.  

Was the extinction of the (non-avian) dinosaurs 65 million years ago a lucky accident necessary for the evolution  of primates and humans?  The question applies to mass extinction events in general, which clearly can significantly alter the course of evolutionary history (a theme of "A New History of Life," by Peter Ward and Joe Kirschvink).  The survivors of such events, however, will typically re-diversify into a range of ecological niches similar to what was present before. This is what convergent evolution shows us. Mammals, in fact,  took over many of the niches left vacant by the dinosaurs.  The actors change, but the roles they play are largely the same. 

As an analogy, think of a village of humans wiped out by some natural disaster, except for one surviving family.  That family first would have to do everything for themselves -  grow food, weave cloth, make candles, etc.  But as their numbers rebounded, they would begin to specialize for different economic roles, eventually specializing into the varied professions that had existed in the previous population. 

As for the dinosaurs, it might have been more difficult for humans to come out of the forest if T. rex had been still stomping around.  However, when considering alternate timelines, or other Earth-like planets, the dynamics between reptiles, mammals, and angiosperm forests could work out in a variety of ways.  Dinosaurs might have died out for other reasons, or if not, been no more of a threat than the large cats encountered by early humans on the savannas. See the interesting BBC post on this topic.

What if humans themselves had gone extinct for some reason? There is a theory that humanity  barely survived an incident that took place some 70,000 years ago.   Some calamity, possibly a super volcanic eruption, is said to have reduced the population of modern humans down to a few thousand.  This is based on DNA evidence suggesting that all existing humans today descended from a single small population.  Without getting into the debate of whether that actually happened, let’s suppose it did, and let's suppose our direct ancestors in Africa had been completely wiped out.  Again, depending on what survived, we might have re-evolved via a different route.  

That particular event apparently didn’t affect Neanderthals, at least not to the same degree, for our rebounding ancestors met them some 20,000 years later as they moved from Africa into the Middle East.  Neanderthals were already quite human, with hunting technology, clothing, and possibly religious beliefs.  They might have picked up the torch if our species in Africa had been wiped out. (Maybe the Klingons were the Neanderthals of another planet!).  Even if Neanderthals had been wiped out too, other surviving primates might have started the process of humanization anew. 

There are other unlikely events cited by the alien civilization nay-sayers, including the origin of life itself, formation of the first eukaryotic cells, and other important steps.  Was the evolution of multicellular animals and the progression from fishes to amphibians to reptiles and mammals inevitable? Would it likely occur the same way on another Earth-like planet? It would take a book to discuss all of these questions, though they could be analyzed in the same evolutionary framework I've used here, with examples of convergent evolution. I think the answers are generally yes.

Of course, all that I have said here is speculative, but my point is that when considering the likelihood of human-like life on other Earth-like planets we must keep in mind the power of natural selection to adapt organisms to new or recurring ecological opportunities.  Convergent evolution, in particular, strongly suggests that particular life forms, or “niches,” will be filled by organisms of different ancestry in different parts of the world (or galaxy), and refilled if emptied by some disaster.  Humans are not the result of a series of lucky accidents, but the product of ordinary evolutionary processes. As cacti are inevitable because of their remarkable ability to survive in a particular environment, so humans might be inevitable because of their remarkable ability to survive and dominate in harsh or unpredictable environments.