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.