Leg bone gives up oldest human DNA ~ Paul RinconScience editor, BBC News.
The discovery of DNA in a 400,000-year-old human thigh bone will open up a new frontier in the study of our ancestors.
That’s the verdict of experts in human evolution on an analysis in Nature journal of the oldest human genetic material ever sequenced.
The femur comes from the famed “Pit of Bones” site in Spain, which gave up the remains of at least 28 ancient people.
But the results are perplexing, raising more questions than answers about our increasingly complex family tree.
The early human remains from the cave site near the northern Spanish city of Burgos have been painstakingly excavated and pieced together over the course of more than two decades by palaeontologists. It has yielded one of the richest assemblages of human bones from this stage of human evolution, in a time called the Middle Pleistocene.
We need all the data we can get to build the whole story of human evolution ~ Prof Chris StringerNatural History Museum
To access the pit, scientists must crawl for hundreds of metres through narrow cave tunnels and rope down through the dark. The bodies were probably deposited there deliberately – their causes of death unknown.
The fossils carry many traits typical of Neanderthals, and either belong to an ancestral species known as Homo heidelbergensis – or, as the British palaeoanthropologist Chris Stringer suggests – are early representatives of the Neanderthal lineage.
DNA’s tendency to break down over time means it has not previously been possible to study the genetics of such ancient members of the human family.
But the recent pace of progress in sequencing technology has astonished many scientists: “Years ago, geneticists said they wouldn’t be able to find DNA that was older than 60,000 years old,” said co-author Jose Bermudez de Castro, and a member of the team that excavated the Pit of Bones fossils.
“Of course, that wasn’t true. The techniques have advanced hugely.”
Siberia to Iberia
Researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, under the supervision of Prof Svante Paabo, have been helping drive those advances.
Prof Paabo, the institute’s director, said: “Our results show that we can now study DNA from human ancestors that are hundreds of thousands of years old,” adding: “It is tremendously exciting.”
The scientists were able to assemble a near-complete sequence of mitochondrial DNA, or mtDNA (the genetic material contained in the tiny “batteries” that power our cells) from the ancient femur. But comparisons of the genetic sequence with those from other ancient and present-day humans yielded a surprise.
Rather than showing a relationship between the Spanish specimens and later Neanderthals, which might be expected based purely on physical features of the fossils and geography, the mitochondrial DNA was most similar to that found in 40,000 year-old fossils unearthed thousands of kilometres away at Denisova Cave in Siberia.
The Denisovans are a genetically distinct group of ancient humans, identified only from genetic material extracted from fragmentary bones at the cave. Their relationship to human groups already described in the fossil record is unclear; they are, as some researchers have remarked, “a genome in search of a fossil”.
From missing mutations in the old DNA sequences, the researchers calculated that the Pit of Bones individual shared a common ancestor with the Denisovans about 700,000 years ago.
Muddle in the middle
So there are several possibilities as to how Denisovan-like DNA could turn up in Middle Pleistocene Spain. Firstly, the mitochondrial DNA type from the pit came from an even more ancient population of humans ancestral to both the Spanish hominids and to Denisovans. This genetic lineage must later have been lost from the Neanderthal gene pool.
Secondly, interbreeding between the Pit of Bones people (or their ancestors) and yet another early human species brought the Denisovan-like DNA into this western population.
Prof Chris Stringer, from London’s Natural History Museum, told BBC News: “We need all the data we can get to build the whole story of human evolution. We can’t just build it from stone tools, we can’t just build it from the fossils. Having the DNA gives us a whole new way of looking at it.”
However, he points out, mtDNA is a small and unusual component of our genetic blueprint, from which only limited conclusions can be drawn. For example, no sign of the interbreeding we now know took place between Neanderthals and modern humans remains in the mtDNA of modern people.
To get the full picture, scientists had to sequence nuclear DNA (that kept in the nuclei of cells) from Neanderthals and compare it with that in present-day populations. Likewise, the true relationships between the Pit people and other ancient populations may only be known if and when nuclear DNA is available.
This will be a challenge given the age of the Spanish fossils, but their good state of preservation – largely a product of the fairly constant temperature inside the cave, gives hope.
“That is our next big here, to sequence at least part of the nuclear genome from the individual in the Sima de los Huesos,” Svante Paabo told BBC News.
“This will answer definitively the question of how they are related to Neanderthals, modern humans and Denisovans.”
- Scientists Just Sequenced the DNA From A 400,000-Year-Old Early Human (blogs.smithsonianmag.com)
- Fossils Yield Oldest Known Human DNA (blogs.discovermagazine.com)
- Discovery of Oldest DNA Scrambles Human Origins Picture (news.nationalgeographic.com)
- Oldest human genome dug up in Spain’s pit of bones (newscientist.com)
- Ancient DNA From Human Relative Sets Age Record (abcnews.go.com)
- Mystery Humans Spiced Up Ancients’ Rampant Sex Lives (nature.com)
- Genetic Analysis Suggests Ancient Humans Interbred Extensively Neanderthals, Denisovans And An “Unknown Population” (rinf.com)
- Oldest human DNA reveals mysterious branch of humanity (sott.net)