08 April 2010


Another relative

Cráneo de Australopithecus sediba at Flickr.comIt's quite astonishing how many fossils of extinct human relatives that paleontologists have found in recent years. Just in the past year, I've mentioned Darwinius and Ardipithecus. And this week we hear about Australopithecus sediba, which sheds further light on how ancient apes transitioned to walking upright like we do.

It's a relief that in the latest coverage, the scientists involved have gone out of their way to say that A. sediba is not a "missing link":

"The 'missing link' made sense when we could take the earliest fossils and the latest ones and line them up in a row. It was easy back then," explained Smithsonian Institution paleontologist Richard Potts. But now researchers know there was great diversity of branches in the human family tree rather than a single smooth line.

All of evolution works that way: branching, somewhat messy relationships between organisms, with many extinct species and a few (or perhaps many) survivors. In the case of hominids, we humans are the only bipedal ones left, and we're also by far the most numerous of the surviving lineage, which includes chimpanzees, bonobos, gorillas, and orangutans too. So our evolutionary past can look linear, at least during the past 5 to 7 million years since our ancestral line diverged from the chimp ancestral line, even though it wasn't.

While we have found quite a few fossils of human relatives, that's a very relative term. Until the past few thousand years, there weren't many of us around at all. All the Australopithecus fossils ever found can be outnumbered by the number of trilobite fossils (which are hundreds of millions of years older!) in a single chunk of stone at any rocks-and-gems store. Hominid fossils are still extremely rare things, so we can't reconstruct the branches of our family tree with perfect accuracy. That's because we can't be sure which (if any) of the species we've unearthed were our direct ancestors, and which were our "cousins"—branches of the tree that died out.

Still, we can make attempts with the data we have, some making more assumptions than others, and the options being quite complex. Yet as we find out more, and discover more, our family tree becomes clearer. That's pretty cool.

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06 March 2010


Ida: now just a nice fossil

Remember the crazy hype last year about "Ida," the beautifully preserved 47 million–year–old primate? The one I called a "cool fossil that got turned into a publicity stunt?"

It turns out that, yes, the original authors seem to have rushed their paper about Ida into publication, presumably in order to meet a deadline for a TV special. And even by the loosest definition of the term, Ida is no "missing link" whatsoever, and not closely related to humans. (Not that relatedness to humans is what should make a fossil important, mind you.)

So now, like Ardi, who's ten times younger, Ida is what it deserves to be: a fascinating set of remains from which we can learn many things, but not anything that fundamentally revolutionizes our understanding of primate evolution. And that's a good thing.

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02 October 2009


Ardi is another fascinating hominid fossil, but "missing link" no longer makes sense

Fosil_Rekaan_Ardi at Flickr.comFirst, let's get something out of the way: the term missing link has long been useless, especially in human palaeontology. That's reinforced by this week's publication of a special issue of the journal Science, focusing on the 4.4-million-year-old skeleton of Ardipithecus ramidus, nicknamed "Ardi."

Chimpanzees are the closest living relative to human beings in the world, but our common ancestor lived a long time ago—at least 6 million years, likely more. When Charles Darwin proposed, some 150 years ago, that the fossil links between humans and chimps were likely to be found in Africa, he was right, but at the time those links really were missing.

It took a few decades until the first Homo erectus fossils were found in Asia. Since then, palaeontologists have found many different skeletons and skeletal parts of Homo, Paranthropus, and Australopithecus, with the oldest, as predicted, all in Africa. We've been filling in our side of the chimp-human family tree for over a century. (The chimpanzee lineage has been more difficult, probably because their ancestors' forest habitats are less prone to preserving fossils—and we're less prone to paying as much attention to it too.)

So it's been a long time since the links were missing, and the term missing link is now more misleading than helpful. Like "Ida," the 47-million-year-old primate revealed with too much hype earlier this year, "Ardi" is no missing link, but she is another fascinating fossil showing interesting things about our ancestry. Most notably, she walked upright as we do, but had a much smaller brain, more like other apes—as well as feet that remained better for climbing trees than ours.

None of that should be shocking. We shouldn't expect any common ancestor of chimps and humans to be more like either one of us, since both lineages have kept evolving these past 6 million years. Ardi isn't a common ancestor—she's more closely related to humans than to chimps, and accordingly shows more human characteristics (or, perhaps better, we show characteristics more like hers), like bipedal walking and flexible wrists. That's extremely cool.

Also interestingly, "Ardi" is not new. It's taken researchers more than 15 years to analyze her, and similar fossils found nearby in Ethiopa's Afar desert. Simply teasing the extremely fragile, powdery fossil materials out of the soil without destroying them took years. Science can sometimes be a painstaking process, not for those who lack patience.

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