Ice Age Siberians

The verified human history of eastern Siberia just got 16,000 years longer. Several hundred miles north of the Arctic Circle, along one of northeast Asia's largest rivers, a team led by Vladimir V. Pitulko, an archaeologist at the Institute for the History of Material Culture in Saint Petersburg, Russia, recently discovered a 30,000-year-old encampment--almost twice as old as the next-oldest known Arctic settlement. The team has unearthed stone tools, animal bones showing signs of butchering and cooking, and spear shafts made from woolly rhinoceros horn and mammoth tusk.

     Plant and pollen remains suggest the region was probably dominated by large expanses of floodplain meadow--an attractive landscape for plant eaters such as bison, hare, and reindeer. Whether the people at the site were permanent residents or seasonal hunters is unclear, but to some investigators their mere presence indicates that people had already habituated to a cold climate. More important, it raises the possibility that people were poised to cross the Bering land bridge to North America earlier than commonly believed--well before the height of the last ice age, 20,000 years ago. ("The Yana RHS site: Humans in the Arctic before the last glacial maximum," Science 303:52-56, January 2, 2004)

Palliative or Poison?

Vultures, the quintessential garbage collectors, were a familiar sight in South Asia just a decade ago--particularly the Oriental white-backed vulture, Gyps bengalensis. But sometime in the 1990s they began dying off at an alarming rate. So J. Lindsay Oaks, a veterinarian at Washington State University in Pullman, and a team of colleagues decided to do a few hundred necropsies in Pakistan. What they found was widespread evidence of acute kidney failure, probably caused by the ingestion of something toxic.

     Arsenic, cadmium, copper, lead, mercury, pesticides, viruses, other infections--you name it, they tested for it and ruled it out. The actual culprit was a substance frequently present in the carcasses of dead livestock, the vultures' normal food source.

     Nowadays, sick buffalo, cattle, and goats are often given diclofenac, a nonsteroidal anti-inflammatory drug that has become available in Pakistan in the past five years. Virtually every veterinarian and drug retailer sells it. Oaks and his team say the drug is the reason not merely for isolated cases of poisoning among scavengers, but for the near disappearance of the entire G. bengalensis species from the Indian subcontinent. (“Diclofenac residues as the cause of vulture population decline in Pakistan,” Nature 427:630–32, February 12, 2004)

Evolutionary Circles

According to Dollo's law--an evolutionary maxim named after the nineteenth-century Belgian paleontologist Louis Dollo, and much favored by the late Stephen Jay Gould--complex physical features lost during evolution are seldom regained. Why? Well, presumably, once a gene no longer gives rise to a characteristic, natural selection no longer exercises quality control on the gene. Eventually the gene gets swamped by mutations. Transformed beyond recognition, it cannot fulfill its earlier function.

     There are three possible exceptions to the pattern: First, after the old gene disappears, a brand-new gene with the same function might evolve elsewhere in the genome. Second, the gene might be co-opted for some other function, thereby remaining sufficiently unchanged to resume its old role later. Third, the gene might remain functional at the larval stage of life, though no longer useful in adulthood. In that case, a simple change in timing could shift the lost feature back from larva to adult.

     Rachel Collin of the Smithsonian Tropical Research Institute in Panama City, Panama, and Roberto Cipriani of Simón Bolívar University in Caracas, Venezuela, have just provided the first good example of the third scenario by reconstructing the evolutionary tree of nearly a hundred species in the limpet family Calyptraeidae. The coiled shell that covered the ancestor of most of those species lost its coiling between 65 million and 99 million years ago, and so most limpet species now have a conical or somewhat hat-shaped shell. But 40 million to 80 million years later, the shell of one species regained the coil. Key to Collin and Cipriani's argument is the fact that many calyptraeid larvae still have a coiled shell, even though the adults do not. (“Dollo's law and the re-evolution of shell coiling,” Proceedings of the Royal Society of London B 270:2551-55, December 22, 2003)

All in the Family

Rafflesia, a plant genus native to the jungles of Southeast Asia, is notorious for its flowers, the largest in the world. A Rafflesia flower can be as broad as three feet across and weigh close to twenty pounds. The plant is also completely parasitic. Lacking leaves, roots, and stems of its own, it depends on its host, a grapevine, for nutrients as well as for water. Except when flowering, it lives inside the vine. Rafflesia's most memorable feature, though, is that its enormous blossom stinks of rotting flesh, the better to lure the carrion flies that pollinate it. What on Earth are its evolutionary origins?

     That mystery may finally have been solved by Todd J. Barkman, a botanist at Western Michigan University in Kalamazoo, and his colleagues, who have devised a family tree built from ninety-five likely relatives. Their approach was based on an analysis of mitochondrial, rather than nuclear, DNA--the latter changes so rapidly in Rafflesia that it proves of little use to genetic studies. Rafflesia's cousins turn out to be such modest blossoms as the passionflower, the poinsettia, and the tiny, fragrant violet; a few relatives are vines. Perhaps, say the investigators, there's something about the close contact between vines and the plants they entwine that fosters parasitism. (“Mitochondrial DNA sequences reveal the photosynthetic relatives of Rafflesia, the world's largest flower,” Proceedings of the National Academy of Sciences 101:787–92, January 20, 2004)

You Take the Muscles, I'll Take the Ears

Geneticists are fond of pointing out that the DNA of people and chimpanzees is nearly 99 percent identical [see “Searching for Your Inner Chimp,” by Carl Zimmer, December 2002/January 2003]. Explaining the manifest differences has become something of a cottage industry. What accounts for the elaborate verbal skills of humans, our technological proficiency, our ability to contemplate our own humanity?

     Now Andrew G. Clark, a population geneticist at Cornell University in Ithaca, New York, and his colleagues have weighed in with a comparative study of more than 7,600 human and chimpanzee gene sequences that have a common origin but have been diverging as the two species have evolved. Any divergence could result from random mutation, but accelerated divergence suggests the operation of natural selection--the emergence and preservation of advantageous characteristics. Clark and his team aimed to identify the latter.

     So what's been briskly diverging? To some extent, the genes that facilitate differing lifestyles. In chimpanzees, rapidly changing genes include the ones that encode embryonic muscle, bone, and connective tissue, as well as skeletal structure in adults. In people, notable modifications have taken place in the genes for smelling and hearing, for instance. Clark and his colleagues speculate that an increasingly fine-tuned sense of hearing has assisted humans in the comprehension of spoken language. (“Inferring nonneutral evolution from human-chimpanzee-mouse orthologous gene trios,” Science 302:1960-63, December 12, 2003)

Feeling Pressured

Several hours before tropical storm Gabrielle struck Florida's Gulf Coast on September 14, 2001, all the juvenile blacktip sharks living in a shallow coastal nursery in Terra Ceia Bay moved to deeper—and safer—waters offshore. Michelle R. Heupel and her colleagues from the Mote Marine Laboratory in Sarasota painstakingly ruled out possible reasons for the sharks' collective departure—the noise of heavy rainfall, the decline in the bay's salinity, increased wind speed, abnormal tides. The only environmental cue that truly coincided with the departure time was a sudden drop in barometric pressure. That finding is the first behavioral evidence that some sharks (and perhaps other coastal species) come equipped with an internal barometer, enabling them to anticipate storms and hurricanes. (“Running before the storm: Blacktip sharks respond to falling barometric pressure associated with Tropical Storm Gabrielle,” Journal of Fish Biology 63:1357–63, November 2003)