News Brief: Ancient teeth point to Neandertal relatives

Modern humans — our species — go back roughly 200,000 years. But over much of their early history they weren’t the only hominids walking the Earth. Among well-known fellow travelers were Neandertals (which died out around 30,000 years ago). A line of hominids closely related to them is known as the Denisovans (Deh-NEE-so-vins). Over time, Neandertals and Denisovans interbred with humans. But no one is quite sure when. So scientists have been quite curious about where early non-human species lived and when. New data now suggest that Denisovans may have emerged as a separate hominid line at least 110,000 years ago.

They interbred with humans in East Asia at least 44,000 years ago. David Reich works in Cambridge, Mass., for both the Broad Institute at MIT and at Harvard University. In 2011, his team reported that some 5 percent of genes in native groups now living in Australia, New Guinea and on several nearby islands trace to Denisovans. This suggested Denisovans must once have been widespread throughout Asia.

But until now, the only purely Denisovan DNA had come from a single finger bone that turned up in 2008. And it wasn’t found in the South Pacific. It came from Denisova Cave (hence the population’s name) in Siberia.

Evolutionary geneticist Susanna Sawyer works for the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. She and her colleagues have now extracted DNA from two Denisovan teeth. Both came from the same Siberian cave.

Her team now concludes that Denisovans must have lived in Siberia long before they had migrated to Australasia. Indeed, the most recent of the Siberian fossils appears to be some 50,000 years old. And an analysis of all three fossils from Denisova Cave suggest that by then, Denisovans already must have been living in the area for some 60,000 years.

What the DNA showed

When people hear the term DNA, they think of the genetic material in the nucleus of nearly every cell. But there’s another form as well. It’s known as mitochondrial (My-toh-KON-dree-ul) DNA. It’s also found in cells, but outside of the nucleus. It resides in structures known as mitochondria, which produce energy for cells.

Sawyer’s group found both types of DNA in the ancient teeth.

DNA undergoes changes over time. They are known as mutations. The mitochondrial DNA in one tooth displayed far fewer mutations than did the DNA from the finger and second tooth. For such a big difference in the genes to have developed within a hominid group, Denisovans must have inhabited this region for roughly 60,000 years. That’s the conclusion that Sawyer’s team presented November 16 in the Proceedings of the National Academy of Sciences.

Nuclear DNA comparisons show that all three specimens from Denisova Cave belonged to a common population. It was distinct from both Neandertals and modern humans. Overall, the genetic diversity seen in fossils from the three Denisovan individuals is greater than has been seen so far in Neandertal fossils. It’s smaller, however, than the genetic diversity in modern humans. That suggests that of these three hominids, our species has now survived the longest.

Power Words

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Australasia Lands in the South Pacific that include Australia, New Zealand, New Guinea and a host of nearby islands.

Denisovans An ancient humanlike population whose existence is known only because of a few fossils discovered in a cave in Siberia.

DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

evolutionary genetics A field of biology that focuses on how genes — and the traits they lead to — change over long periods of time (potentially over millennia or more). People who work in this field are known as evolutionary geneticists.

gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

hominid A primate of an animal family that includes humans and their fossil ancestors.

mitochondria (sing. mitochondrion) A structure in all cells (except bacteria) found outside of their nuclei. Here the cell breaks down nutrients and converts them into a form of energy known as ATP.

mitochondrial DNA DNA passed on to offspring, almost always by their female parent. Housed in mitochrondria, this DNA is double-stranded but circular. It’s also very small, only possessing a small share of the genes found in the main package of DNA, the material found in a cell’s nucleus.

mutation Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.

Neandertal A species (Homo neanderthalensis) that lived in Europe and parts of Asia from about 200,000 years ago to roughly 28,000 years ago.

nucleus Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information.

Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal’s content spans the biological, physical, and social sciences. Each of the more than 3,000 papers published each year, now, not only is peer reviewed but also approved by a member of the U.S. National Academy of Sciences.

Siberia A region in northern Asia, almost all of which falls within Russia. This land takes its name from thelanguage of the Tatar people, where Siber means sleeping land. This region is vast. It has become famous for its long, harsh winters, where temperatures can fall to −68° Celsius (−90° Fahrenheit).

Stone Age A prehistoric period, lasting millions of years and ending tens of thousands of years ago, when weapons and tools were made of stone or of materials such as bone, wood, or horn.