Ancient iceman has no modern kin
Posted by Jennifer Evans
[Entry posted at 30th October 2008 05:01 PM GMT]
http://www.the-scientist.com/blog/display/55145/
The 5,000-year-old mummy Öetzi, found in a glacier in the European alps 17 years ago and believed to be an ancestor of modern Europeans, actually belonged to a different genetic family and may have no living descendants, researchers report today in Current Biology.
The researchers sequenced mitochondrial DNA extracted from Öetzi's intestines, offering the oldest complete mtDNA sequence of modern humans.
"We sort of assume when we look at populations today we see representations of [ancient populations] as well," Joanna Mountain an anthropological geneticist at Stanford University who was not involved in the study, told The Scientist. The current study, she said, "counters that thinking."
"Sequences from mummies and fossils can inspire us to consider a whole new history or path for human history" by revealing populations that we don't commonly think about, she added.
Franco Rollo, an anthropological geneticist at the University of Camerino in Italy and first author of the study, began studying the now-famous iceman only a few days after Öetzi was pulled from an Alpine glacier in 1991. Since then, Rollo and other researchers have studied Öetzi extensively, from the fabric of the clothes he wore to the last meal he ate. In their latest study Rollo and colleagues sequenced mtDNA obtained from Öetzi's intestines.
An organism's DNA starts to degrade immediately after death, so studying ancient samples can be tricky, Martin Richards, an archeogeneticist at the University of Leeds and study coauthor explained to The Scientist. Compared to chromosomal DNA, mtDNA is a smaller molecule that occurs in higher concentration in cells. Because mtDNA is passed along the maternal line, forgoing recombination, and has a high mutation rate, it provides a good model for studying human evolution, Richards said.
Preliminary sequencing studies of roughly 400 base-pairs of mtDNA acquired from bone conducted in 1994 led researchers to believe modern day Europeans who share a common ancestral DNA sequence, called the K1 lineage, were descendants of Öetzi.
With the DNA samples from Öetzi's intestines, Rollo's group used PCR amplification and pyrosequencing technology to reassemble and sequence Öetzi's entire 16,569 base-pair mtDNA genome. The researchers then compared Öetzi's mtDNA genome to a database of 115 modern people in the K1 lineage. Although Öetzi shared a mutation with modern people in the K1 lineage, three additional mutations suggested Öetzi's mtDNA belonged to a different sublineage group than modern Europeans.
The results "show the iceman belongs to a cluster found in Europe [the K1 lineage], but he belongs to a branch that appears to have diverged from the others ... some 20,000 years ago ... and seems to have become very rare," if not extinct, Richards said.
The study raises the questions about Öetzi's clade, Mountain said. "Was the clade rare at the time [Öetzi lived] ... or did entire populations [like Öetzi] go extinct that were once common?"
"Öetzi is the evidence of an evolutionary process that makes some mitochondrial lines surviv[e] through the millennia while other are lost," Rollo told The Scientist by Email. "One wonders what we could find if we were able to study more ancient H. sapiens remains."
Because of issues of rapid degradation and contamination in studying ancient DNA, Richards explained, scientists are "stuck with trying to work out modern genetic sequences and inferring back [in time]," limiting understanding of human evolution to only the lineages that survived not those who went extinct.
Rollo's team now plans to attempt the sequencing of Öetzi's Y chromosome DNA, which is less abundant than mtDNA, but offers a counterpart to mtDNA, in that it is passed only through the paternal line.
"It will be also very interesting to see whether the descendents of Öetzi indeed are all extinct," Rollo said. "[As far as we know], no one can claim to be the descendent of Öetzi but, who knows, perhaps in a lonely Alpine valley..." |
