I’m an Ape, and I’m Also a Fish

Humans are hominoids. I know that sounds redundant. Blame anthropologists for a lack of taxonomic imagination. But the fact that I am a hominoid, I believe, is a significant fact. In everyday terms, it means that I am an ape.

Years ago, when my elementary school teachers delineated the tree of life in biology class, apes and humans were kept separate from each other. We belonged in our own group, the Hominidae, and the so-called great apes – orangutans, gorillas, and chimpanzees – formed another group, the Pongidae. The two groups were thought to have diverged from a common, primitive ape ancestor. But a combination of genetic and fossil discoveries changed this traditional view. Chimpanzees turned out to be our closest living relatives, with gorillas and orangutans on the next proximal branches to the group containing both us and Pan. Our family was not separate from the ape lineage. We are one kind of highly-intelligent, specialized ape.

In an essay published last week, written in response to a piece by evolutionary biologist Jerry Coyne, anthropologist John Hawks called the statement that we are apes “a canard.” Ape is a vernacular English term, Hawks argues, and therefore the word should only be applied to gibbons, orangutans, gorillas, and chimpanzees, but not us. To call humans apes, he says, is an act of “Orwellian coercion” meant to debase our cherished and easily-bruised sense of self-importance. We can say that we’re hominoids or hominids – those are appropriate technical terms – but Hawks would rather we leave “ape” well enough alone.

Hawks doesn’t allow comments on his blog, but, thankfully, the discussion spilled over onto Twitter and other blogs. Historian of science John Wilkins articulated a response to Hawks faster than I could. (I have about ten days before the first draft of my next book is due, so I’m a bit slow in keeping up with internet kerfuffles.) There is no impenetrable wall between technical terms and popular usage, or, as Wilkins wrote, “Experts introduce and revise terms that the folk pick up.” Wilkins uses the word “dinosaur”, and the recently-altered meaning of the term, as an example. The anatomist Richard Owen coined “dinosaur” in 1842, and the term trickled out into public understanding to represent big, fierce, and otherwise monstrous prehistoric reptiles. But since the late 1990s, at the very latest, birds have been recognized as dinosaur descendants, and, in a very real sense, are themselves dinosaurs. This altered understanding – in which there are avian and non-avian dinosaurs – is beginning to take hold. Whether born in popular or academic circles, terms change meaning according to our mutating view of Nature.

In the same way, the way we understand what an ape is has changed. A revised evolutionary picture is influencing the way we apply the word. I don’t share Hawks’ frustration over this point. In fact, I think such statements – which seem to fly in the face of what we previously believed to be true – help people ask questions about how organisms are related to each other. Saying “humans are apes” or “birds are dinosaurs” still sounds strange enough that such statements demand evidence and provide opportunities for engagement, in addition to being a reflection of recent revisions to evolutionary trees.

And the words we choose depend upon how specific we wish to be. In an evolutionary context, I am simultaneously an ape, a monkey, a primate, a mammal, a therapsid, a synapsid, an amniote, a tetrapod, and, to pick an arbitrary stopping point that suits this post’s purpose, a fish. You are a fish, too. Now, I typically don’t come home from an afternoon walk and tell my wife “There were so many fish walking around the park. Everyone’s out today” – such a statement would make it sound as if I had slipped into a Ray Troll painting – but, in an evolutionary sense, it still would have been true. Among other things, we’re fish. The term isn’t terribly specific, but it’s not inaccurate, either, as a newly-announced cousin of ours demonstrates.

The origin of the first vertebrates capable of crawling on land was one of the most important events in our evolutionary history. These creatures are known as tetrapods by virtue of having four (tetra) limbs, and a growing number of discoveries has begun to outline how fleshy-finned fish were adapted into the first amphibious vertebrates. Tiktaalik, a roughly 375 million year old “fishapod”, is the most famous of such creatures – the fish bears a suite of transitional features intermediate between those of more archaic fish and the earliest true tetrapods. But Tiktaalik is not alone. Other vertebrates, such as the vaguely salamander-like Ventastega and the flattened Panderichthys, also demonstrate that our bodies, as Neil Shubin so wonderfully articulated in Your Inner Fish, are modified from archaic fishy forms. And University of California, Berkeley paleontologist Brian Swartz has just described another creature relevant to this famous evolutionary event.

Named Tinirau clackae, the fossil creature at the center of Swartz’s study was not an early landlubber. Tinirau was a fully aquatic fish, and a fish quite similar to the iconic, fleshy-finned form Eusthenopteron (a prehistoric fish once believed to have been the starting point for tetrapod evolution). As Swartz points out, though, the relationships of these creatures to the next grade in the evolutionary transition – flattened fish with stouter, more limb-like fins such as Tiktaalik – is not entirely clear. Tinirau adds a little more resolution to the picture.

The six fossils of this fish, originally collected in the 1970s from roughly 387 million year old deposits Nevada, look like bony smears across rock slabs. But, in detail, much of the fish remain intact. The skull, a significant part of the backbone, and several sets of bones which supported the fins of Tinirau are visible in one of the better specimens, designated UCMP 118605. Those fin bones are especially important. Rather than being arrays of splint-like bones – as you can see in a perch, bass, or many other fish – the fin bones correspond to the bones in our own arms. The fins attached to the body by way of a single bone – the equivalent of the humeri in our arms and femora in our legs – and archaic precursors of our lower arm and leg bones can be seen in the collection of fin bones below. Over 380 million years ago, the basic form of our limbs was already in place, albeit in fish which swam through the Devonian sea.

In Swartz’s analysis, Tinirau came out relatively close to Panderichthys and Tiktaalik. While Tinirau probably was not a direct ancestor of either form, the fish still represents the bauplan from which the antecedents of the first true tetrapods evolved. Exactly when the first tetrapod with distinguished fingers and toes evolved, however, is another matter. Two years ago paleontologist Grzegorz Niedźwiedzki and colleagues described 395 million year old tracks from Poland which might have been made by tetrapods with differentiated digits. The tracks predate the earliest known body fossils of such creatures by about 20 million years.

There is more than one possible answer for the discrepancy. Perhaps there is a 20 million year record of early tetrapods that we simply have not uncovered yet. Then again, maybe the tracks were not made by tetrapods at all – impressions and tracks created by invertebrates have often been confused for the footprints of early tetrapods. And Swartz offers another possibility. Coelacanths and lungfish – fleshy-finned fish that are modern cousins of Tinirau – can move their stubby fins in alternating patterns resembling a walk, and modern lungfish which walk over soft sediments are capable of creating the same kinds of tracks Niedźwiedzki and co-authors described. The supposed tetrapod tracks may have been made by more archaic fish which used their fins to walk.

I don’t expect the idea that we are fish to pick up much popular currency. The everyday, paraphyletic meaning of the term is entrenched, and I don’t expect anyone to refer to the salmon in their sushi as a “non-tetrapodomorph fish.” But the idea is still a useful one as we explore our relationship to the rest of life on earth. After all, we share a common ancestry with every other living thing on the planet, and, for a time, our ancestors and kin were snake-like fish with thick fins supported by stacks of bone. The way those fish swam, and walked, through prehistoric seas formed the foundation for the flowering of vertebrate evolution on land, including the later origin of a lonely species of upright ape obsessed with its own beginnings.

References:

Schwartz, B. (2012). A Marine Stem-Tetrapod from the Devonian of Western North America PLoS One : 10.1371/journal.pone.0033683