Saturday, May 15, 2010

Kissing Cousins - Opinionator Blog

Olivia JudsonOlivia Judson on the influence of science and biology on modern life.

The past comes to us in tantalizing fragments — a bone here, a footprint there. But of all the fragments yet discovered, perhaps none is so tantalizing as the one published in the journal Science last week: the Neanderthal genome.

Neanderthals have perplexed and intrigued us ever since the first bones were discovered in a cave in what is now Germany, in 1856. Who were they? Why did they vanish?

Neanderthal and human skeletonsJames Estrin/The New York Times A reproduction of a Neanderthal skeleton, left, and a modern Homo sapiens skeleton, right.

Over the past century and a half, our picture of them has become less blurry, more distinct. From their bones we know that Neanderthals were bigger and stronger than us “anatomically modern humans,” and they had larger skulls that boasted prominent eyebrow ridges. They appear to be the descendants of a lineage that separated from ours around 400,000 years ago, wandered out of Africa, and lived across Europe and central Asia. The last of the Neanderthals lived on the Iberian peninsula, dying out sometime between 37,000 and 28,000 years ago.

(Anatomically modern humans, in contrast, evolved in Africa, arriving at recognizably modern skeletons between 130,000 and 200,000 years ago. Some time later — 65,000 years ago or so — a group of them left Africa, wending their way through the Middle East and across Eurasia, the Pacific and the Americas. These were the ancestors of today’s non-African populations; and in Europe and central Asia, they coexisted with Neanderthals until the Neanderthals disappeared.)

What else do we know about Neanderthals? They may have decorated their bodies with ornaments; they certainly used tools like axes and spears. They hunted. Indeed, they mostly seem to have eaten meat — they are sometimes described as “top carnivores” — and because of their bigness, probably needed more calories per day than we do.

As our ability to retrieve and sequence ancient DNA has developed and improved, we’ve been able to paint in further details. Some Neanderthals may have had pale skin and red hair. Some of them could taste bitter flavors. They may have had a capacity for speech, though we can’t tell if they had much in the way of language.

And now, with the full genome sequence, we can start to answer many more questions, both about Neanderthals and about ourselves. The idea is that if you line up the sequences of humans, Neanderthals and chimpanzees, you can start to trace which genetic changes occurred when. Unsurprisingly, the data suggest that by far the bulk of our genetic evolution happened in the millions of years before humans and Neanderthals separated; the handful of known differences between us and Neanderthals occur in a motley ragbag of genes. (There’s no obvious stamp of rapid brain evolution, for example.)

The sequence is an amazing accomplishment. Yes, it’s preliminary and contains plenty of errors. But think of this: the DNA was extracted from bones that are tens of thousands of years old. Whereas the DNA in your cells is present in nice long strings, in ancient specimens it’s broken into tiny fragments, if it’s preserved at all. Then there’s the problem of DNA swamping. Which is to say that more than 95 percent of the DNA extracted from the bones belongs to microbes that lived on the bones in the subsequent millennia; this had to be stripped out. Ditto, the DNA from any humans who have handled those bones. As one of my colleagues remarked, the “methods” section of the paper reads like a molecular obstacle course. To have any useable DNA at all, let alone a full genome, is astonishing. Hats off.

And the results stoke the imagination, for they provide more evidence for something that has long been suspected: Neanderthals are not just a quirky sideshow in human evolution, but an intimate part of our own story. Many of us have Neanderthals in our family tree, just as some of us have Hottentots, or Aztecs, or Genghis Khan.

Which isn’t surprising. To be sure, Neanderthals were more genetically distinct from us than any living humans are from one another. But they are still our close relatives — kissing cousins, if you will—and when closely related beings meet, they often take a shine to each other. Coyotes, for example, sometimes cavort with dogs or wolves. Geoffroy’s cat, a south American pussy, sometimes gallivants with another local wildcat, the oncilla, even though their lineages separated a million years ago — much longer ago than ours split from Neanderthals. And ducks of many kinds seem to like mating with one another. Our ancestors, it seems, were no different.

All the same, the idea of Neanderthal ancestry brings a vividness to the distant past. Were the men exotic and sexy? What were half-Neanderthal, half-human children like? Were they extra-beautiful, as people with mixed ancestries often are? Did they have an unusual hungering for red meat? Did we learn Neanderthal customs, or languages?

And it brings a greater poignancy to that other mystery — why did the Neanderthals vanish?

Here, lots of ideas have been put forward — a sure sign that no one knows. Perhaps they died of mad Neanderthal disease, owing to a habit of feasting on one another’s brains. (This has been put forward as a serious hypothesis.) Perhaps they were victims of a changing climate. Perhaps they were “inferior” beings, unable to match our capacity for innovation in the face of adversity. Perhaps their populations became too small, and too sparse, for them to find mates. Or — and this is the most haunting possibility — perhaps they were eventually murdered by their puny cousins. That is, us.

Notes:

For the Neanderthal genome (and a complex lesson in how to extract Neanderthal DNA), see Green, R. E. et al. 2010. “A draft sequence of the Neandertal genome.” Science 328: 710-722. This paper also provides evidence for human-Neanderthal interbreeding. For a more detailed look at human-Neanderthal differences, see Burbano, H. A. et al. 2010. “Targeted investigation of the Neandertal genome by array-based sequence capture.” Science 328: 723-725.

For a fascinating account of Neanderthal bone structure, see Sawyer, G. J. and Maley, B. 2005. “Neanderthal reconstructed.” Anatomical Record 283B: 23-31.

Working out what happened when in human history is a complex and approximate business. I took the date of 400,000 years since the separation of humans and Neanderthals from the Burbano paper mentioned above. Exactly when Neanderthals disappeared from Europe is disputed. For the 28,000 years ago claim, see Finlayson, C. et al. 2006. “Late survival of Neanderthals at the southernmost extreme of Europe.” Nature 443: 850-853. For the claim that the real date is 37,000 years ago, see Zilhão, J. et al. 2010. “Pego do Diabo (Loures, Portugal): dating the emergence of anatomical modernity in westernmost Eurasia.” PLoS One 5: e8880. The dates I give for anatomically modern humans are approximate, but within the range that is generally accepted; see, for example, Fagundes, N. J. R. et al. 2007. “Statistical evaluation of alternative models of human evolution.” Proceedings of the National Academy of Sciences USA 104: 17614-17619; see also the references listed therein.

Whether or not Neanderthals wore jewelry is vigorously contested; for evidence that they did, and a discussion of why some people think they didn’t, see, for example, Zilhão, J. et al. 2010. “Symbolic use of marine shells and mineral pigments by Iberian Neandertals.” Proceedings of the National Academy of Sciences USA 107: 1023-1028. For evidence that Neanderthals mostly ate meat and count as “top carnivores,” see Richards, M. P. and Trinkaus, E. 2009. “Isotopic evidence for the diets of European Neanderthals and early modern humans.” Proceedings of the National Academy of Sciences USA 106: 16034-16039. (These authors also suggest that an inability to switch diets might somehow have led to the Neanderthal extinction.) For Neanderthals and tools, see for example SantaMaría, D. et al. 2010. “The technological and typological behaviour of a Neanderthal group from El Sidrón Cave (Asturias, Spain).” Oxford Journal of Archaeology 29: 119-148.

For red-headed Neanderthals, see Lalueza-Fox, C. et al. 2007. “A melanocortin 1 receptor allele suggests varying pigmentation among Neanderthals.” Science 318: 1453-1455. For their ability to taste bitterness, see Lalueza-Fox, C. et al. 2009. “Bitter taste perception in Neanderthals through the analysis of the TAS2R38 gene.” Biology Letters 5: 809-811. For a possible linguistic capacity, see Krause, J. et al. 2007. “The derived FOXP2 variant of modern humans was shared with Neanderthals.” Current Biology 17: 1908-1912.

For earlier suspicions that Neanderthals and humans interbred see, for example, Trinkaus, E. 2007. “European early modern humans and the fate of the Neandertals.” Proceedings of the National Academy of Sciences USA 104: 7367-7372; and Herrera, K. J. et al. 2009. “To what extent did Neanderthals and modern humans interact?” Biological Reviews 84: 245-257.

For coyotes cavorting with wolves, see Kays, R., Curtis, A. and Kirchman, J. J. 2010. “Rapid adaptive evolution of northeastern coyotes via hybridization with wolves.” Biological Letters 6: 89-93. For coyotes and dogs, see Adams, J. R., Leonard, J. A., and Waits, L. P. 2003. “Widespread occurrence of a domestic dog mitochondrial DNA haplotype in southeastern US coyotes.” Molecular Ecology 12: 541-546. For hanky-panky in south American cats, see Trigo, T. C. et al. 2008. “Inter-species hybrization among Neotropical cats of the genus Leopardus, and evidence for an introgressive hybrid zone between L. geoffroyi and L. tigrinus in southern Brazil.” Molecular Ecology 17: 4317-4333. A an overview of similar goings-on in ducks can be found in Muñoz-Fuentes, V. et al. 2007. “Hybridization between white-headed ducks and introduced ruddy ducks in Spain.” Molecular Ecology 16: 629-638.

The suggestion that mad Neanderthal disease caused their demise has been put forward several times; see, for example, Cooper, J. H. 2000. “Did cannibalism and spongiform encephalopathy contribute to the demise of the Neanderthals?” Mankind Quarterly 41: 175-180 and Underdown, S. 2008. “A potential role for transmissible spongiform encephalopathies in Neanderthal extinction.” Medical Hypotheses 71: 4-7. The notion that Neanderthals were culturally inferior to us — and that this caused their extinction — is pervasive; see, for example, Klein, R. G. 2003. “Whither the Neanderthals?” Science 299: 1525-1527. For the possibility that small, sparse populations was the eventual problem, see Hublin, J.-J. and Roebroeks, W. 2009. “Ebb and flow or regional extinctions? On the character of Neandertal occupation of northern environments.” Comptes Rendus Palevol 8: 503-509. (This paper is also my source for the claim that Neanderthals needed to eat more calories than we do.) For other hypotheses, see Herrera, K. J. et al. 2009. “To what extent did Neanderthals and modern humans interact?” Biological Reviews 84: 245-257.

Many thanks to Thiago Carvalho, Mike Eisen, Gideon Lichfield and Jonathan Swire for insights, comments and suggestions.

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