By Elof Carlson
Fossils are relatively rare because most of the animals and plants that have died in nature have been eaten or decomposed. Fossils are often found in sedimentary rocks, and those dead organisms were buried after drowning, caught by volcanic ash, buried in a mudslide or sucked down by quicksand or some other event less likely than falling on a field or in the underbrush of a forest, or left as scattered bones by hungry predators. Only in the past few ten thousand years have humans buried their dead, improving the chances that their remains will someday be unearthed and studied by paleontologists.
Until the last half of the twentieth century, the only way to use human fossils to work out a historical association was through comparative anatomy and a variety of chemical and physical tools to determine the age of the sediments in which they were unearthed. The idea of a paleogenetics arose in 1963, with the use of that term by Linus Pauling and his colleagues, who studied the amino acid sequences in hemoglobin molecules of numerous organisms, from sipunculoid worms to humans, that use hemoglobin to carry oxygen to body tissues.
In 1964, the first sequence of fragments of the DNA of an extinct quagga were worked out using the skin of an extinct specimen in a museum. The quagga was an animal that looked like a chimera of giraffe and a zebra.
Once DNA sequencing was worked out, especially by Fred Sanger and his colleagues, viruses, bacteria, single-celled organisms, and then more complex worms and flies were sequenced. By 2000, the human genome was being worked out. Svante Pääbo and his colleagues are leaders in the working out of fossil human DNA.
This is what has been found so far. Four contenders for species status lived about 40,000 years ago. Three populations of humans arose after an initial origin in Africa. Of these three, the Neandertals (Homo neanderthalis) left Africa earlier than our own Homo sapiens. The Neandertals were named for the Neander river valley where they found in Germany. We were named by Linnaeus as Man (Homo) the Thinker (sapiens).
Two additional populations were found, one in western Siberia and the other in Indonesia. The Siberian humans are called Denisovans (Homo denisova). They were named for the Denis cave in which they were found and they also had an exit from Africa. The Indonesian humans are called Homo floresiensis and are named for the island Flores in Indonesia where they were found. Where they came from is not yet known. They are unusual for their small size, a Hobbit-like three- and-a-half feet tall.
The DNAs of three forms of humanity have been sequenced. The complete sequence of DNA of an organism’s cell is called a genome. The Indonesian form went extinct about 12,000 years ago, but no DNA has been extracted from their remains. Neandertals and Denisovans went extinct about 40,000 years ago.
Analysis of the three available genomes shows that most Europeans have about 4 percent Neandertal DNA. Living people in Melanesia and Australian aborigines have about 4 percent H. denisova DNA. About 17 percent of Denisovan DNA is from Neandertals. The human branch Homo bifurcated and one branch split into H. neanderthalis and H. denisova. The other branch from Homo produced us, H. sapiens. We are 99.7 percent alike for H. sapiens and H. neanderthalis.
Since we have 3 billion nucleotides to our genome, there remain 9 million mutations between us, most of it in our junk DNA. There are, nevertheless, hundreds of gene differences between our two species. It also means that where these populations came into contact, fertile matings occurred, and remain in our DNA from our ancestral “kissing cousins.”
Elof Axel Carlson is a distinguished teaching professor emeritus in the Department of Biochemistry and Cell Biology at Stony Brook University.