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Ardipithecus ramidus October 4, 2009

Posted by rationalskeptic in Fossils.
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In its 2 October 2009 issue, Science presents 11 papers, authored by a diverse international team, describing an early hominid species, Ardipithecus ramidus, and its environment. These 4.4 million year old hominid fossils sit within a critical early part of human evolution, and cast new and sometimes surprising light on the evolution of human limbs and locomotion, the habitats occupied by early hominids, and the nature of our last common ancestor with chimps. (from sciencemag.org AAAS)

This is an area of study that I love to talk about and post about, but I am going to leave this post up to the experts to explain:  Here is a portion of Carl Zimmer’s post over at his blog The Loom.

Nice Guys With Little Teeth

Those of you reading this post that have a Y chromosome have canine teeth that are about the same size as those of my XX readers. The same rule applies to the teeth of some other primate species. But in still other species, the males have much bigger canines than the females. The difference corresponds fairly well to the kind of social lives these primates have. Big canines are a sign of intense competition between males. Canine teeth in some primate species get honed into sharp daggers that males can use as weapons in battles for territory and for the opportunity to mate with females.

Men have stubby canines, which many scientists take as a sign that the competition between males became less intense in our hominid lineage. That was likely due to a shift in family life. Male chimpanzees compete with each other to mate with females, but they don’t help with the kids when they’re born. Humans form long-term bonds, with fathers helping mothers by, for example, getting more food for the kids to eat. There’s still male-male competition in our lineage, but it’s a lot less intense than in other species.

White and his colleagues  found so many teeth of different Ardipithecus individuals that they could compare male and female canines with some confidence. The male teeth turn out to be surprisingly blunted. This result suggests that hominids shifted away from a typical ape social structure early in our ancestry. If this was a result of males forming long-term bonds with females and helping raise young, this shift was able to occur while hominids were still living a very ape-like life. Ardipithecus existed about 2 million years before the oldest evidence of stone tools, suggesting that technology was not the trigger for the evolution of nice hominid guys.

Walking, Of A Sort; And Climbing, Of A Sort

C. Owen Lovejoy of Kent State University spearheaded the studies on how Ardipithecus moved. He and his colleagues argue that its pelvis could support its upper body during bipedal walking. It wasn’t a fabulous walker, and was probably a terrible runner. Nevertheless, it had some of the same anchors for muscles that we have on our pelvis, and which chimpanzees and other apes lack. Its pelvis was, in other words, a mosaic. Lucy, we now can see, represents a later step in the journey towards out own walking-adapted anatomy.ardipithecus side view440

Ardipithecus’s feet were mosaics too. The four little toes were adapted for walking on the ground. Yet the big toe was still opposable, much like our thumbs. This sort of big toe helped Ardipithecus move through the trees much more adeptly than Lucy.

But Ardipithecus could not climb through trees as well as, say, chimpanzees. Chimpanzees have lots of adaptations in their arms and shoulders to let them hang from branches and climb vertically up trees with incredible speed. Ardipithecus had hands were not stiffened enough to let them move like chimpanzees. Ardipithecus probably moved carefully through the trees, using its hands and feet all at once to grip branches.

Just a Reminder: We Didn’t Evolve From Chimpanzees

Chimpanzees may be our closest living relatives, but that doesn’t mean that our common ancestor with them looked precisely like a chimp. In fact, a lot of what makes a chimpanzee a chimpanzee evolved after our two lineages split roughly 7 million years ago. Ardipithecus offers strong evidence for the newness of chimps.

Only after our ancestors branched off from chimpanzees, Lovejoy and his colleagues argue, did chimpanzee arms evolve the right shape for swinging through trees. Chimpanzee arms are also adapted for knuckle-walking, while Ardipithecus didn’t have the right anatomy to lean comfortably on their hands. Chimpanzees also have peculiar adaptations in their feet that make them particularly adept in trees. For example, they’re missing a bone found in monkeys and humans, which helps to stiffen our feet. The lack of this bone makes chimpanzee feet even more flexible in trees, but it also makes them worse at walking on the ground. Ardipithecus had that same foot bone we have. This pattern suggests that chimpanzees lost the bone after their split with our ancestors, becoming even better at tree-climbing.

Chimpanzees do still tell us certain things about our ancestry. Our ancestors had chimp-sized brains. They were hairy like chimps and other apes. And like chimps, they didn’t wear jewelry or play the trumpet.

But then again, humans turn out to be a good stand-in for the ancestors of chimpanzees in some ways–now that Ardipithecus has clambered finally into view.

Check out all the links in this post to read through the commentary and the actual research itself.



1. tim - October 12, 2009

It doesnt make sense for an animal of that time period to not be able to run or climb well. This animal would be an easy target for any predator. How could it survive with these type of defensive weapons at its disposal?

rationalskeptic - October 12, 2009

Considering Ardipithecus is extinct it makes complete sense.

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