On the Rotatability of Evolutionary Branches, or On Life’s Little Joke, or On Why We Ain’t Special

“It is obvious to common sense that some organisms are higher than others – that a dog is higher than his fleas, or a fish higher than a jellyfish.” – Julian Huxley, in Evolutionary Humanism

It may be common sense, but common sense isn’t always right.

The most rampant misconceptions of how evolution works all coincide with how we humans perceive ourselves. Many believe we are the ultimate goal of evolution, that our existence is inevitable, and that we are superior to all other species – we are perched at the top of the ladder: the Ascent of Man.

The omnipresent and endlessly irritating Ascent of Man. At least it admits to the more humble origins of our species...

It is difficult to not agree that we are better than all other species. After all, we are the self-reflecting and intelligent species that reached the moon, right? The domination by our lineage has lasted millions of years as we eliminated competitors like the Neanderthals, domesticated dozens of species like wolves, aurochs, and maize, and have doomed countless others to their extinction.

We are certainly a successful species depending on your metric, but are humans a part of a successful lineage? Think about it: There are no other living members of our genus Homo! (The Neanderthals and Homo erectus are long dead.) Other members of our subtribe, the hominins, which includes the australopithecines (of Lucy fame), have gone extinct too. While Homo sapiens is successful, our lineage as a whole has not been.

The human family tree is rather like a bush, but only a single twig at the top, Homo sapiens, remains. Source: AMNH.

The famed biologist and science popularizer Stephen Jay Gould called this phenomenon “Life’s Little Joke.” While he framed it around horses, it applies equally to humans.

This is life’s little joke. By imposing the model of the ladder upon the reality of bushes, we have guaranteed that our classic examples of evolutionary progress can only apply to unsuccessful lineages on the very brink of extermination – for we can linearize a bush only if it maintains but one surviving twig that we can falsely place at the summit of a ladder.

Would we consider ourselves the greatest species, and our arrival, inevitable, if Neanderthals and the robust austalopithecines were still alive? While the human lineage today survives as a lone branch, what would we think if it were a bush with multiple branches instead? Such a world is difficult to imagine. But as Gould argues, we consider ourselves successful because our nearest cousins are dead.

This self-congratulating perception is in stark contrast to other actually successful mammalian lineages. Can you name a greatest bat or rodent? Or even the greatest member of the genus Panthera? Is it the lion, tiger, jaguar, or leopard? There is no clear choice. We can also ask a similar question of the ape family: Which is the greatest ape?

The apes are composed of two families, Hylobatidae and Hominidae. Hylobatidae contains 14 extant species of gibbon and Hominidae contains a measly five species – the orangutan, gorilla, chimp, bonobo, and human.* (Compare this with the mouse & rat family Muridae which contains over 600 species!) Not even our family is that successful!

How are these five species related? Well, the Ascent of Man implies that there was a linear ascension from chimps to humans, and this is a popular misconception, but it is incredibly false. Here is how the actual relationships are properly represented in a branched evolutionary tree.

gibbon - orangutan - gorilla - chimp - bonobo - human

Unfortunately while this tree is correct, it lends itself to further misinterpretations. Because there is a straight line from the root to humans, people may believe that this image tells us that humans are the most advanced species and that we are the end result of evolution because we are the furthest to the right and those inferior to us lie to the left. If you only look at the photos and ignore the tree below, the Ascent of Man is actually recreated. Again, this couldn’t be more false.

Now look at the tree itself. The intersection between the lines connecting the baby human and baby chimps (chimp + bonobo) is called a node. This node represents the most recent common ancestor between us and the chimps. Imagine your family tree and designate chimps and bonobos as siblings and chimps and humans as cousins. In this scenario, just as you share a grandparent (represented by a node here) with your cousin, we share a “grandparent” with the chimps!** Chimps did not become humans!

Furthermore, what we are looking at is the relationships between living species. Most twigs and branches are dead and hidden. If we were to magnify the tree, we would see the australopithecines, Homo habilis and other extinct ancestors and create a bushier tree, much like the figure above this one.

Because the node represents the common ancestor of chimps and humans, the order of the two actually doesn’t matter. Just as in a family tree where you and your siblings can switch places beneath your parents and it wouldn’t matter, the twigs on the tree can also be swapped. (Note that chimps and bonobos are two species in one genus and can also swap places.)

gibbon - orangutan - gorilla - human - chimp - bonobo

This tree tells us the exact same thing as the previous tree. The information has not changed, but what it means has. When we rotate the branches, the tree places humans in the middle of the other apes. The misconceptions previously discussed fall apart when viewed in this light. Are chimps and bonobos the end result of evolution? Did chimps evolve from humans? No! We are all just tips of branches of a tree, evolutionarily equal, and those tips are surrounded by other tips.

gibbon - orangutan - human - chimp - bonobo - gorilla

The same information with a different layout leads to a different interpretation.

When Huxley says there are higher and lower species, he is making a non-scientific value judgment. Evolutionary theory teaches us that there really is no higher or lower. We are not inherently superior to cats, mice or fleas, nor to chimps and gorillas. We are all twigs upon a bush, equidistant in time from the last common ancestor of all life. To say one species is better than another is to choose a specific trait at the expense of all others. We may be the most intelligent, for example, but we are not particularly quick, reproductive, or virulent, and damn it, I wish I could fly!

As Gould’s little joke tells us, we only consider ourselves successful because those that were most like us, the Neanderthals, were killed many years ago. If Neanderthals were bigger, stronger, and smarter than us, and still alive, we would probably be running an inferiority complex instead. There are others like us alive today though, the other great apes, and their intelligence and emotional capacitance should not go unappreciated. I mean, those babies look damn similar to each other!

So are we the ultimate goal of evolution? No.

Were we inevitable? No.

Are we better than all other species? No.

Did we evolve from chimps? No, we share a common ancestor with them; we are cousins, not fathers & sons.

Who are we then?

We are apes who evolved from other apes. We are all twigs upon a bush.

I need hardly remind everybody that at least one other mammalian lineage, preeminent among all in our attention and concern, shares with horses the sorry state of reduction from a formerly luxuriant bush to a single surviving twig – the very property of extreme tenuousness that permits us to build a ladder reaching only to the heart of our own folly and hubris. – Stephen Jay Gould


* For some embarrassing reason, I was not aware that there were two species of gorillas and two species of orangutans – I had thought there were only subspecies. Funnily enough, this adds to Life’s Little Joke: Homo is the only hominin genus that has only a single extant species! – the rest have two (with multiple subspecies). I may fix my evolutionary tree at some point, but not right now.

** The grandparent metaphor breaks down when you realize that the most recent common ancestor of chimps and humans would have actually been a population of individuals, not a single pair. Population thinking is difficult for people to wrap their heads around, I think, but it is a concept we should teach. “Mitochondrial Eve” lends itself to many a misinterpretation, for example. Had I thought of it at the time of writing, I probably would have addressed this and took care of that misconception as well!

Stephen Jay Gould, Life’s Little Joke. PDF

Links to original photos:

5 thoughts on “On the Rotatability of Evolutionary Branches, or On Life’s Little Joke, or On Why We Ain’t Special

  1. Are we better than all other species? No.

    That’s a non-scientific value judgment, too. Why do you say no? What do you mean by better? Surely there are many things we are better at than all other species…

    • Hmm, fair point. By “better,” I mean a generic superiority. It isn’t a rigorous definition, but it’s one I think people frequently use. I would use “superior” to be more exact, I think, though.

      As I said in the post, we may be more intelligent than any other species, but that is choosing a single trait. We are not particularly quick on our feet nor can we change our body plan from an amoeboid to a flagellate. We like intelligence, of course, and it is an incredible trait, but we are being choosy. Isn’t it reasonable to say that there is no inherent superiority between two (especially non-competing) species, or that it is meaningless to even ask the question?

      Perhaps we could use absolute fitness to determine which species are biologically superior to another species, a competition in which the virus might “win” but that may be susceptible to the same problem of being picky.

      I am not necessarily set in stone on my answer; just thinking it through.

      • I think you’re underplaying sapience. It allows us to be biologically inferior in countless ways, yet allows us to be Superior in those same ways and more.

        Myopia is a pretty good example. Near sightedness is no longer a biological end, it is easily rendered moot and if we take it a step farther we can readily view objects just larger than an atom, and peer billions of light years into space. Heck we can even peer through objects.

        That being said, you have a very good thought experiment. You say we’re a twig on a bush, but we’re the only twig that can truly shape the bush.

  2. “We’re the only twig that can truly shape the bush” – really? Think viruses, bacteria. They’ve shaped humans quite a bit, and made us what we are, and are not, today.

    • That is a good point, given the ambiguity of my statement. I would say that I meant humans are the only organisms that can intentionally shape the tree of life – we can decide to rescue peregrine falcons from extinction, for example.

      Your comment reminded me of this quote from evolutionary biologist R.A. Fisher on the issue of “creativity” in evolution though, which you may find of interest:

      From “Creative Aspects of Natural Law” (1950):

      “Just where does the theory of natural selection place the creative causes which shape evolutionary change? In the actual life of living things; in their contacts and conflicts with their environments, with the outer world as it is to them; in their unconscious efforts to grow, or their more conscious efforts to move. Especially, in the vital drama of the success or failure of each of their enterprises. …Creative causation should be a function of every organ through entire life history, of the brain in devising, and of the hands in execution, than that it should be confined to the fertilised ovum. To the selectionist it acts, not in the dark by potentiality only, but by real effects in the real world.

      The theory of Selection seems to me also holistic … in the mutual reaction of each organism with the whole ecological situation in which it lives—the creative action of one species on another. The timid antelope has played its part in the creation of the lion, and species long extinct must have left indelible memorials in their effects on species still surviving. Who knows if the mammals would ever have evolved, but for the creative activity of the dinosaurs!”

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