Scientists have discovered what is believed to be the oldest human DNA in biological history.
A 400,000-year old human thigh bone was discovered in Spain and has left the scientists baffled about human evolution.
The discovery was reported in the journal Nature on Wednesday, December 3, and the fossil was discovered in the caves of Sierra de Atapuerca, Spain. The retrieval of the oldest human DNA ever sequenced breaks the previous record, which dates back to 100,000 years.
The femur was found at the well-known "Pit of Bones" site in Spain, which has, till now, thrown up remains of nearly 28 individuals.
"Excavations of a complex of caves in the Sierra de Atapuerca in northern Spain have unearthed hominin fossils that range in age from the early Pleistocene to the Holocene1. One of these sites, the 'Sima de los Huesos' ('pit of bones'), has yielded the world's largest assemblage of Middle Pleistocene hominin fossils consisting of at least 28 individuals dated to over 300,000 years ago," reports Nature.
To access the pit, researchers need to crawl for hundreds of meters via the narrow cave tunnels and rope down in the dark. A possibility exists that the bodies were deposited in the pit intentionally, but the cause of death is not known.
The discovery of the oldest DNA, however, has raised several questions about the complex human family tree and the evolution process. The thigh bone was initially thought to belong to forerunners of Neanderthals, but the DNA resembles that of Denisovans.
The analysis of the bones has raised questions over the established ideologies regarding the geographical spread of our the early humans, the Neanderthals and Denisovans. Until the recent finding, these sister families of the early humans were believed to have resided in Europe (Neanderthals) and Siberia (Denisovans).
However, in a new study, paleontologists opine that the bones of what they believed to be European Neanderthals "appear genetically closer to the Siberian Denisovans," which is reflected by maternally inherited "mitochondrial" DNA that was found in the recently discovered femur in Spain.
"The fact that they show a mitochondrial genome sequence similar to that of Denisovans is irritating," said Matthias Meyer of Germany's Max Planck Institute for Evolutionary Anthropology in Leipzig, lead author of the study. "Our results suggest that the evolutionary history of Neanderthals and Denisovans may be very complicated and possibly involved mixing between different archaic human groups."
Before the modern-day humans evolved and spread worldwide from Africa, the Neanderthals and Denisovans walked the Earth thousands of years ago. The minor traces of their genetic code that is found in modern humans are reflective of the interbreeding among ancient humans.
According to Meyer, the stable temperatures in the cave may have aided in preserving the mitochondrial DNA. He also lauded the recent scientific advancements in gene-sequencing technology for helping establish the new milestone.
Per scientists, the fossils may belong to an ancient species called Homo heidelbergensis or are early representatives of the Neanderthal lineage.
Since DNA tends to break down over a period of time, it has not been possible to study the genetic history of these early humans. However, the recent sequencing technology has made major breakthroughs.
"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, from the National Research Centre for Human Evolution (CENIEH), a member of the team that excavated the fossils. "Of course, that wasn't true. The techniques have advanced hugely."
