Denisovans and others

Just over a year ago, I wrote a piece about the 2012 discovery of a group of early hominins, the Denisovans, from a cave in Siberia. All that was initially found was a finger bone, from which was extracted some DNA, which showed the owner as not belonging to the Neanderthals. This finger bone belonged to the first group of archaic humans to be revealed solely by their DNA and associated proteins, rather than the morphology of their fossilised bones. Subsequently another sample of Denisovan DNA was extracted from another specimen from the same cave1. Because the group was only ‘defined’ from DNA, it has been jokingly referred to as ‘a genome in search of a fossil record’2.

In 2021, Chinese researchers published a paper detailing an ancient skull that they suspected belonged to a previously unknown species of human. The specimen was supposedly found in northeast China in 1933 during the Japanese occupation of the area and was subsequently hidden. It only came to the attention of scientists in 2018. Nicknamed ‘Dragon Man’, the specimen represents a human group that lived in East Asia at least 146,000 years ago2.

Because the skull was removed from its surrounding sediment and no record of the locality was kept, it has no ‘archaeological context’, so that it is unknown how it lived and what tools it used. However, because of the completeness of the specimen and its clear difference from any other hominin skulls, it was assigned a new species name in the 2021 paper; Homo longi3.

The origin of modern humans (Homo sapiens) has long been controversial. During the late Middle and Late Pleistocene, (~1 million to 100,000 years ago) several human lineages, maybe at species level, coexisted with H. sapiens across Africa and Eurasia. These extinct hominins include H. heidelbergensis, Homo naledi, Homo floresiensis, H. luzonensis, Denisovans, Neanderthals (Homo neanderthalensis), and H. erectus. The phylogenetic relationship between these coexisting hominins and H. sapiens has long been debated3.

Numerous archaic hominin fossils from Asia show a combination of features present in H. erectus, H. heidelbergensis, H. neanderthalensis and H. sapiens. Therefore, it is likely that these Asian hominins are critical for studying the later evolution of the genus Homo and the origin of H. sapiens4.

Ancient DNA has revealed a great deal about our history, especially interbreeding events with archaic hominins. However, DNA degrades much faster than other biomolecules such as proteins and it is these which may enhance our ability to study remains in which DNA is not preserved. Proteins from the ‘Dragon Man’ skull have been analysed and over 90 proteins have been identified and they indicate that this specimen is most closely related to the Denisovans5. If this is confirmed, Denisovans could be assigned to Homo longi.

Back when I first learned about human evolution from the How and Why book of ‘Primitive Man’6, published in 1961, while I was an inquisitive child, there were relatively very few specimens of fossil hominins, as it was only two years beforehand that the Leakeys had started discovering hominin fossils in Olduvai Gorge in Tanzania7. At that time, while there was still debate about whether Neanderthals belonged to the same species as us, they were clearly considered to be something different from us. In more recent times, there have been many, many more hominin fossils discovered and they have been assigned to numerous genera and many species8.

This has led to a bit of a crisis in the understanding of what constitutes a hominin species. Some years ago, this led me to ask, partly flippantly, if Linnaeus got it wrong when he created the binomial system of classifying life9. This caused angst among some palaeontologists who told me I didn’t understand what Linnaeus was trying to do. I thought this was funny, in that I have operated under the Linnean binomial system all my professional life and will continue to do so because there is nothing better on offer. The species I study are morphospecies; i.e. species determined by their morphology alone, as they are long extinct groups, and are too old to provide any DNA. There have been numerous attempts to define species, none of them entirely successful. The one definition which many zoologists seemed to favour was the ‘biological species’ concept which was introduced by Ernst Mayr in the 1940s10.

This asserted that a species was reproductively isolated from any other species such that if two species interbred, the offspring would be infertile, with the mule (offspring of a female horse and a male donkey) being the infertile example most often provided. However, the ‘biological species’ concept has serious deficiencies in that numerous cases of supposedly different species mating with others to produce fertile offspring are numerous, and in the case of human evolution have been demonstrated by DNA extracted from the original Denisovan finger bone. Roughly 3-5% of Denisovan DNA is found in Melanesians, Australian Aboriginals and Tibetans, while the genome of modern Europeans and Asians contains about 1-4% of Neanderthal DNA. In addition, one of the Denisovan specimens from Denisova cave contained DNA that indicated the specimen belonged to a young girl, one of whose parents was a Denisovan, while the other was a Neanderthal11. Genomic research has also shown that many species of vertebrates interbreed with other species, and it has been estimated that as much as 16% of bird species interbreed with another species12.

Thus, the problem is not with other species that interbreed with each other, but with the biological species concept. This concept was a zoological concept and, as such, didn’t deal with the fourth dimension; that of time. As I say above, in palaeontology, we only deal with morphospecies. It seems that zoology will have to resort to that species concept too, something that palaeoanthropologist Chris Stringer has suggested. He noted that measurement of braincases and pelvic shape can reliably separate modern humans from Neanderthals. The latter’s fossils have a longer, lower skull and a wider pelvis, and even the three tiny middle ear bones of our middle ear can be readily distinguished from those of Neanderthals with careful measurement13.

A large part of science is about classifying things, and classifications, as their name implies, means that the things scientists study have to be put into classes. Unfortunately, nature doesn’t always allow itself to be put into such boxes, as much of it is characterised by continua.

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Denisovans and others