Prehistoric DNA to Help Solve Human-Evolution Mysteries?
John Pickrell in England
for National Geographic News
|March 25, 2004|
It may be possible to extract DNA from the bones of human ancestors and other hominids who died up to one million years ago, researchers believe. Hominids are primates that walk upright, including humans and extinct human ancestors and related forms.
Experts speaking at a chemistry conference held in Chicago earlier this month argued that ancient genetic material could be used to better understand the relationships among hominids and answer questions about the evolution of speech and other defining traits of humans.
"DNA is a relatively weak molecule, comparatively speaking, yet under certain conditions it persists in the fossil record despite what chemistry [in the lab] predicts," said Hendrik Poinar, a molecular anthropologist at McMaster University in Ontario Canada. Fragments of genetic material may survive much longer in fossils than laboratory experiments have so far predicted, he said.
Revolution in Evolution Studies
The study of ancient human evolution is one of science's most contentious disciplines. Anthropologists are frequently locked in debate on issues ranging from migration to classification of hominid species. However, new molecular techniques may now revolutionize the field, as well as the study of ancient plants and other animals.
DNA extracted from specimens of extinct animals has already been used to show that the Mauritian dodo is a close cousin to the common pigeon. It has also proven that widely varying bones left behind by New Zealand's giant moa birds belong to the massive females and much-smaller males of a single species. Previous analysis of bones alone had led researchers to mistakenly classify the un-sexed remains into a large number of different species.
The key to finding ancient DNA lies in the conditions of preservation, Poinar said. Researchers already know that DNA is relatively easy to extract from tens-of-thousands-of-years-old mammoths buried in Siberian permafrost. But desert caves with constant temperatures and very low levels of humidity can also be surprisingly good at preservation, he said.
In contrast, horse bones left in the open in a moist temperate country such as Germany might be completely stripped of genetic material in less than 50 years.
Laboratory experiments that estimate the rate of degradation of DNA in bone have predicted that large fragments of the molecule are unable to survive longer than 10,000 years in temperate regions, Poinar said. But researchers have already managed to find DNA sequences that have survived much longer.
Poinar led the team that last year extracted DNA from 30,000-year-old fossilized ground sloth dung. That DNA had persisted against all odds in a warm Nevada desert cave and was four times as old as the theoretical age limit predicted for DNA survival at that temperature.
Protein attached to bone can also hold genetic data (protein sequences mirror those of the DNA they are modeled on), and these more robust molecules may persist even longer in the fossil record.
British researchers revealed in 2002 that they had extracted the world's first intact protein sequence from 60,000-year-old frozen bison bones. As yet unpublished findings may soon reveal protein sequences from horses many times older, Poinar said.
The oldest DNA ever found comes from Siberian dirt analyzed by bioarchaeologist Eske Willerslev at Oxford University in England. Willerslev, who also spoke at PITTCON, the Chicago chemistry conference, has found fragments of DNA from plants, fungi, and animals in 350,000-year-old permafrost soil cores.
But Poinar believes that, under exceptional circumstances, researchers might be able to get small fragments of DNA in human and animal bones that are up to a million years old, and protein sequences from even earlier.
Hominid DNA could then be used to piece together the much-disputed relationships of our extinct relatives.
"Sadly there's not much hope for many of the African fossils," Poinar said, pointing to the fact that most hominids are known from hot climates where DNA degrades rapidly. Cool cave sites at high elevations would be the best bet, he said.
Poinar's work centers on the analysis of coprolites: fossilized human and animal dung. Analyzing DNA from fossilized Cro-Magnon human and Neandertal feces might provide answers to questions about early human evolution, particularly the evolution of language. Poinar hopes to acquire fossil coprolites from the caves of Mount Carmel near Haifa in Israel.
Geneticists have shown that a gene called FOXP2 may be required for the fine-tuning of speech. Studies suggest that this gene may have evolved in its present version around 50,000 years ago, Poinar said.
DNA in these fossils from Israel might therefore confirm whether the modern version of the gene had evolved by that time and might hint at whether Neandertalsthe last of whom died out 40,000 years agohad the ability to speak or not.
"In theory it might be possible to extract hominid DNA from [very old] fossil bones," commented Matthew Collins, director of York University's Bioarcheology Center in England. Collins is also a speaker at the Chicago conference and part of the team that extracted protein from the 60,000-year-old bison bone.
But proteins are much more durable, Collins said. Tightly bound to bone, and permanently frozen, they might last for an astounding 100 million years. Though a very long shot, he said, it is theoretically possible that even dinosaur fossils (the youngest of which are 60 million years old) could harbor some genetic information.
For more DNA and fossil news, scroll down for related stories and links.
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