This article was originally posted at Science 2.0 on 9 October 2009. It provides some background for the follow-up article corREXion? that has suddenly become relevant again.
Paleontology doesn’t always get the respect it deserves (or desires), in the molecular, genomic, evolutionary, quantitative genetic circles we run in around here. Blame the DNA. Sequence comparisons have proven incomparable in establishing phylogenetic relationships between organisms.
Paleontology can also irritate us by creating false controversy, which irritates the heck out of us. The fossil record is a sparse and biased record of life. Supposed “missing links” are often an artifact of this fact. Supposed discrepancies between sequence divergence times and divergence in form from the fossil record often reflect the fact that sequence divergence necessarily precedes any differences in form significant enough to be noticeable in the fossil record.
This means that biologists tend to relegate paleontology (fairly or not) into the roles of adding colorful detail. Therefore, it is particularly exciting when there are not one, not two, but three recently reported fossils that force the biological community to re-evaluate evolutionary hypotheses.
Pre-Island Gigantism: Komodo Dragons
Komodo dragons, the largest living lizards, were considered iconic examples of island gigantism. When we discovered the extinct giant monitor lizard Megalania in Australia, Komodo dragons became candidates for examples of insular dwarfism.
Now, discoveries of Komodo dragon remains in Australia predating (300,000-4 million years ago) any from their current island homes (900,000 years ago) suggest that Komodo dragons developed their current, impressive size on the continent and then radiated out to the islands. Subsequent changes in evolutionary pressures may have contributed to the lizards going extinct in Australia, but surviving on the islands. Unfortunately for the island gigantism/dwarfism hypotheses, isolation on islands does not appear to have driven changes in the size of the lizards.
The Scalable Tyrant: Raptorex
It was thought that the peculiar characteristics of Tyranosaurus rex – a huge head and reduced, two-fingered forelimbs cantilevered over long, straight, running legs and balanced by a massive tail – evolved slowly with increased body size. A recently described specimen puts the lie to those assumptions. Despite being 60 million years older than T. rex and 1:100th the size, Raptorex has all the trademark characteristics of the later giant. This indicates that these characteristics were not adaptations to increased body size, but represented an effective body plan that was scalable to any size. In fact, the physics of cantilevering (e.g., teeter-totters), unlike the physics of flight, scale linearly, which supports this conclusion.
And what we can now say is that this is a body blueprint for a predator, jaws on legs as it were, that is one of the most successful of the Mesozoic.
-Paul Sereno, describing author
Raptorex also throws a monkey wrench into assumptions made about the evolution of form drawn from our observations of development. Because different body parts grow at different rates, especially during the juvenile period of development (e.g., baby heads and limbs are not in the same proportions as in adults), it was assumed that body part sizes evolve at different rates. While this may still be true in general, Raptorex demonstrates that it is possible to scale-up an effective body plan by orders of magnitude of total size over evolutionary time.
Putting the “Common” Back in Common Ancestor: Ardi
Saying that humans evolved from chimps, like the common creationist characterization of evolution, is a good way to get yourself slapped by an evolutionary biologist. The dirty secret is, however, that even biologists tend to think of this question in the sense of evolving out chimps. We frame the question in terms of “what genes make us human” as if there were some set of mutations that occurred that caused some chimpanzee mama to pop out a slightly more hominidish baby. We have tended that the common ancestor of chimpanzees and hominids would look like a chimpanzee. Enter Ardi.
Ok, technically, this isn’t so completely new, but it has made a big splash recently. Ardipithecus ramidus has been known for years, but we’ve never had such a complete specimen (especially one including pelvis, hands, and feet) as Ardi. A. ramidus and Ardi go back about 4.4 million years, over a million years earlier than Lucy (Australopithicus afarensis) and about 1.5 million years after the consensus chimpanzee-hominid split. Ardi was bipedal on the ground, quadrepedal in trees, lived in the forest, and had less specialied teeth than modern chimps or humans. Oh yeah, we also now think that chimpanzees and gorillas evolved knuckle-walking separately. All together, this implies that the last common ancestor of chimpanzees and hominids didn’t look that much like either of the modern versions.
So, as it turns out, chimpanzees are just as “evolved” to their environment as we humans are to ours. Maybe the fact that there are two extant species of chimpanzee, the Common and the Bonobo (or Hippie Chimp, as I like to call them, due to their tendency to resolve disputes with love, literally and physically).
Over the past few weeks, paleontology has demonstrated its true value. As is frequently said in discussions of scientific philosophy, no amount of evidence can definitively prove a hypothesis, but only a single contrary example can destroy it. The Komodo dragons of Australia, Raptorex, and Ardi are incredibly compelling contrary examples. Bravo.