Apr 24 2009

Pinniped Evolution

The joke is getting so overused now it is becoming a cliche in skeptical circles – what happens when a paleontologist fills in a gap in the fossil record? They create two gaps, one on each side. But it is often used because it pithily exposes the intellectual buffoonery of those evolution deniers (aka creationists) who deny common descent. What is a “gap;” how big does it have to be to call into question common decent; or rather how small do the gaps have to shrink before creationists will accept common descent?

Perhaps the biggest outright lie in the creationist camp, still frequently parroted, is that there is a lack of transitional fossils in the fossil record. That is why it is important to showcase to the public the steady stream of beautiful transitional fossils that are being added to our already copious fossil record.

In the most recent issue of Nature, scientist present yet another pesky gap filled in with a transitional fossil, this one an early pinniped – which includes seals, sealions, and walruses.

The fossil is between 20-24 million years old and is dubbed Puijila darwini. Here is the technical description from the Nature article.

The new taxon retains a long tail and the proportions of its fore- and hindlimbs are more similar to those of modern terrestrial carnivores than to modern pinnipeds. Morphological traits indicative of semi-aquatic adaptation include a forelimb with a prominent deltopectoral ridge on the humerus, a posterodorsally expanded scapula, a pelvis with relatively short ilium, a shortened femur and flattened phalanges, suggestive of webbing.

What this means is that the creature was able to walk on land, was likely a carnivore, but had some early adaptations to the water, such as webbed feat. Think of an otter (it was 110 cm long) with a long tale and the teeth of a dog.  The earliest pinniped fossils come from 20-28 million years ago, about the same time as this fossil, and already have fully developed flippers.

This fossil suggests answers to several unknowns – what evolutionary path did pinnipeds take, what are their closest relatives, and where greographically did their evolution take place? This fossil suggests they evolved in the fresh waters of the arctic, as opposed to the the northwestern US, where the earliest pinniped fossils were found. This one fossil does not settle this last question, but does suggest the arctic as a viable alternative.

I can anticipate the standard creationist denial. They will argue that this fossil cannot be a direct ancestor to pinnipeds because it is as old, and not older, than the earliest pinniped fossils with fully formed flippers.  This is true, as the authors of the Nature article readily state. Most fossils will not be direct ancestors to living descendants. This is because evolutionary relationships are bushy – they are not a ladder of linear progression. A randomly discovered fossil is therefore likely to be on a side branch, not one that lead directly to species that happen to be extant.

The same is true of Archaeopteryx – the transitional species between small theropod dinosaurs and modern birds. Archaeopteryx is almost certainly not on the main line that led to birds. But it does show that in the right time and place there were feathered dinosaurs that had half reptilian and half bird-like morphology. Evolutionary theory also predicted that we would find more – and we have, in spades. There is now a fairly full assortment of feathered dinosaurs at different stages of evolution from dinosaurs to birds, filling in the gaps on either side of archaeopteryx.

The same was true of Ambulocetus, the walking whale. This one species is clearly transitional from terrestrial mammals to whales, but was also adapted to it current niche. It didn’t “know” that its clade was evolving toward whalenss, it was just adapted to its current lifestyle. We can only put it into perspective today – as a transitional species, because we know what some of its ancestors became. And, as with Archaeopteryx, we have since found numerous other fossils filling in the gaps on either side.

We could also look at Australopithecus robustus. This was not the first extinct hominid discovered, but let us say this was the only fossil homonid we had. It is clearly transitional between humans and modern apes, suggesting that we share a common ancestor. But it is also clearly not a direct ancestor of modern humans. It sits on an evolutionary branch that exists in the morphological space between humans and chimpanzees, but is not on the branch that leads directly to modern humans.

But in reality we have many homonid fossil species, and we have a pretty good idea of those whose direct ancestors are modern humans.

Creationists rhetoric is often clueless about the nature of speciation. They argue as if they think that one species turns into another, but ignore the fact that one species can turn into two or more through speciation – separated populations taking different paths. They are therefore puzzled that (taking this new fossil for example) a slightly older fossil can look more like modern pinnipeds than this new fossil. They ignore the fact that the definition of “modern” or “advanced” is relative – in this case relative to modern pinnipeds. When one common ancestor species splits into two, one of which leads to modern pinnipeds and the other does not, the ones on their way to becomming modern pinnipeds will look more like their modern descendants than those on the branch that did not evolve in that direction. So species on the side branch (again, “side” relative to the line that lead to extant species), may retain features that are “primitive” in the extant line.

To further clarify this point (because it is so often confused by creationists), evolution predicts that we will find fossils that are morphologically roughly transitional between extant species with a common ancestor and at a time and place in the past where this evolution was taking place. But it does not predict a strict linear sequence because speciation creates a complex and bushy picture of evolutoinary relationships.

But what has happened, and is happening, in each of the examples I gave and many others, is that as more and more fossil are found they fit nicely into a reasonable tree of relatedness, temporally and morphologically. Evolution predicts that something like Puijila darwini had to exist, and at about the age that it is. Creationism does not. It further predicts that we will find more fossils that flesh out pinniped evolution even further.

Creation “scientist” Duane Gish famously quipped in his many debates with scientists that the notion of terrestrial mammals evolving into whales was like a cow taking to the water, a strategy he characterized as an “utter” failure. Nice pun, but intellectually vacuous. He should have been thinking of otters, not utters. There are already known many living species with various degrees of adaptation to aquatic life. Otters and beavers are comfortable on land, but are good swimmers.  Seals and sealions are able to awkwardly shuffle about on land, but are even more adpated to the sea, with flippers instead of feet. Manatees are similar to walruses, but are one more notch further adapted to the sea and lost their ability to amble out onto land. And whales are fully adapated to aquatic life.

These creatures do not represent and evolutionary sequence, but they do show a plausible sequence of variable degrees of adaptation to the water. And we have been finding fossil species that show a similar sequence, with walking whales and now walking seals.

The fossil evidence for common descent is so stunning in its confirmation of evolutionary theory, that I cannot help wonder about the cognitive dissonance it creates in any creationist even modestly familiar with the evidence.

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