This article [attached] summarizes a talk at a human genomics/evolution meeting in July 2024 in Puerto Vallarta, Mexico, so the data are not yet published. The title of the article — “Oldest human genome comes from a Denisovan” — is a bit confusing (i.e., is Denisovan a “human”?), so perhaps a bit of background is important to get everyone on the same page.
Before “archaic” Homo sapiens, there were at least 21 different “fits” and starts” in southeast Africa, over ~5 million years, before a human-like line emerged and was successfully sustained. Archaic Homo sapiens evolved into Neanderthals and Denisovans (called Homo heidelbergensis) from a common ancestor, which is believed to have lived in Africa between 700,000 and 200,000 years ago. Around 300,000 years ago, one group of Homo heidelbergensis left Africa and split into two groups. Homo erectus (living in Africa between 1.89 million and 110,000 years ago) has also been suggested as being more closely related to Denisovans than Neanderthals.
Homo neanderthalensis moved north and then west into Europe, and (geographically isolated for many thousands of years), they evolved into Neanderthals. The first Neanderthal fossils were discovered in Germany in 1856. Neanderthals developed unique adaptations to survive in cold environments, such as short limbs and torsos to conserve heat, and wide noses to warm and humidify air [This is an example of gene-environment interactions]. Another example of gene-environment interactions would be the very small (height 3½ ft tall) Homo floresiensis, who apparently evolved (over at least 200,000 years) from Homo erectus. Small size appears to be due to living on the small island of Flores, Indonesia (Flores elephants are also tiny in size).
Homo denisovans moved east into Asia and became Denisovans. Denisovans left remnants throughout Asia, and appear to have been more perceptibly preserved in Oceania and parts of Southeast Asia towards the end of the Ice Age.
During the latter part of the 19th century and the early 20th century, fossils that resembled Neanderthals were discovered in (what is now) Belgium, Croatia, France, Italy, Hungary, Israel, Czechia, the Crimea, Uzbekistan, Iraq, the Netherlands, Greece, Syria, Spain, and Russian Siberia. These fossils range from nearly 400,000 years ago or earlier, to as recent as 24,000 years before present. Severe cold conditions existed during the Late Pleistocene (between 129,000 and 11,700 years ago). The complete disappearance of the Neanderthals corresponds to, or precedes, the most recent glacial maximum in the late Pleistocene — a time period of intense cold spells and frequent fluctuations in temperature beginning around 29,000 years ago or earlier — and the increasing presence and density in Eurasia of early modern human populations, and possibly their hunting dogs, beginning as early as 40,000 years ago. For example, around 24,000 years ago, present-day Lake Erie was covered with ice a mile thick.
Homo sapiens remained in Africa until ~100,000 years ago, when they began migrating out. Modern humans and Denisovans likely first met in Eurasia around 40,000 to 60,000 years ago. During the Late Pleistocene (between 129,000 and 11,700 years ago), modern humans interbred with both Neanderthals and Denisovans, and many modern humans today carry DNA from these archaic populations. Some African populations carry almost no Neanderthal DNA, while those from European or Asian backgrounds have 1% to 2%. Denisovan DNA is barely detectable in most parts of the world, but comprises 4% to 6% of the DNA in people in Melanesia (which extends from New Guinea to the Fiji Islands). Scientists have also identified some Denisovan DNA in living humans, including Papuans and Han Chinese people, acquired from past interbreeding.
Using the percent distance from the human–chimpanzee last common ancestor (~6 to 7 million years ago) as a reference, the Denisovan/Neanderthal ancestor is estimated to have diverged from the Homo sapiens (modern human) ancestor ~800,000 years ago, and from each other ~640,000 years ago (while everyone was still in Africa). Although Denisovans and Neanderthals apparently interbred repeatedly (around the edges of their respective territories), their lineages are distinct — acquiring roughly 300,000 genetic changes that differentiate them from Neanderthals, according to the latest DNA sequencing of these genomes.
The attached article largely confirms hanky-panky interbreeding between Neanderthals and Denisovans. The previous “oldest DNA sequence of archaic humans” was from a Neanderthal 120,000 years ago. This talk described successful DNA sequencing from a 200,000-year-old Denisovan. Both genomes provide evidence of interbreeding between these two archaic human sublines, and other studies have confirmed interbreeding of Homo sapiens with both of these archaic sublines…
DwN
Science (19 Jul 2024) 385: pp 240-241