December 5, 2013
Earliest Known Human DNA Offers Clues To Our Origins
[ Watch the Video: Oldest DNA Ever Sequenced Raises Some Questions ]
redOrbit Staff & Wire Reports - Your Universe Online
Geneticists led by Matthias Meyer at the Max Planck Institute for Evolutionary Anthropology in Germany extracted a nearly 400,000-year-old human thigh-bone fossil from the bottom of a cave shaft in northern Spain called Sima de los Huesos, where remains of 28 early humans of an unknown species have been discovered.
The scientists determined a nearly complete mitochondrial genome sequence of the fossil, and found that it was unlike that of Neanderthals, yet also different from that of more modern humans. Instead, the findings confirmed it is representative of the genus Homo, but shares a common ancestor with the Denisovans, extinct relatives of Neanderthals in Asia.
Mitochondrial DNA, or mtDNA, is a small part of the genome that is passed down along the maternal line and occurs in many copies per cell.
“The fact that the mtDNA of the Sima de los Huesos hominin shares a common ancestor with Denisovan rather than Neanderthal mtDNAs is unexpected since its skeletal remains carry Neanderthal-derived features,” Meyer said in a statement posted on the Institute’s website.
Indeed, based on the appearance of the fossilized bones, these primitive precursors to modern humans likely resembled Neanderthals, the researchers said in a paper about the findings published in the journal Nature.
The Denisovans were unknown to science until just a few years ago, when their DNA was first identified from the finger of a young female discovered in a Siberian cave. Her remains dated to about 40,000 years ago, providing evidence that Neanderthals, anatomically modern humans and Denisovans coexisted during that time.
Given their age and Neanderthal-like features, Meyer said the fossilized Sima hominins found in northern Spain were likely related to the population ancestral to both Neanderthals and Denisovans. Another possibility is that gene flow from yet another group of hominins brought the Denisovan-like mtDNA into the Sima hominins or their ancestors.
Meyer’s findings have some experts in human origins puzzled about human ancestral relationships.
"We are lifting the edge of the curtain onto something more complex than any of us had ever thought,” said Tattersall, who was not involved in the current work.
Experts have offered four speculative theories that may account for the findings: people at the Sima cave could have been a distinct species that interbred with Denisovans, they may have been related to the ancestors of Denisovans, or to the ancestors of both Denisovans and Neanderthals, or to an older human species that lived elsewhere in Europe and Asia, such as Homo heidelbergensis.
Or all of those theories may be true.
"I would speculate the likelihood of interbreeding is quite high," Meyer told The Journal.
Indeed, a 2010 genetic analysis of ancient and modern genomes found that there is a tiny bit of Neanderthal DNA in every modern non-African today. And last year, researchers at the Max Planck Institute reported that as much as six percent of the genomes of some modern South Pacific Islanders are Denisovan.
"This tells us something interesting about how our species evolved," population geneticist Michael Hammer, at the University of Arizona, told The Journal. "It may be that interbreeding was a common process in all of human evolution."
Juan-Luis Arsuaga, director of the Center for Contemporary Human Evolution at the Institute of Health Carlos III in Spain, who has worked at the Sima site for 30 years, was also surprised at the current findings.
"We thought they were Neanderthal relatives but the mitochondrial genome told a different story,” he said. "It was unexpected and shocking."
The results point to a “complex pattern of evolution in the origin of Neanderthals and modern humans,” he said. “I hope that more research will help clarify the genetic relationships of the hominins from Sima de los Huesos to Neanderthals and Denisovans.”
The Max Plank researchers are now pursuing this goal by focusing on retrieving DNA from more individuals from the site in Spain, and on retrieving nuclear DNA sequences.
"That is our next big thing here, to sequence at least part of the nuclear genome from the individual in the Sima de los Huesos," said Max Plank Institute director Svante Pääbo in an interview with BBC News. "This will answer definitively the question of how they are related to Neanderthals, modern humans and Denisovans."
Meyer noted that the Sima cave offers nearly perfect conditions for preservation of the fragile molecules that make up DNA.
Under most circumstances, this genetic material quickly breaks down and degrades, but the cool, humid and undisturbed air of the Sima cave shaft offer nearly perfect conditions for their protection, he said.
"It is the most perfect fridge you could build to preserve ancient fossil DNA,” Meyer concluded.