Although neanderthals died out over 40,000 years ago, we are living in an age in which there are more people with their DNA than at any other time in history, with research finding that all modern human beings have neanderthal DNA in them. Homo neanderthalensis evolved some 400,000 years ago, and in their time on earth, they interbred with our ancestors in waves out of and into Africa. Recent evidence shows that this process of hybridizing between ancient humans and neanderthals and other groups was a constant feature of ancient life. 

The Old Scientific Consensus

As professors Katerina Harvati, and Rebecca R. Ackermann noted in a recent paper for Nature Ecology and Evolution, scientific consensus about the evolution of man has been that adaptations, behavioral and biological, have been made over time, leading to the present configuration of humanity as homo sapiens. Part of that adaptation involves waves of migration out of Africa as humanity propagated itself across the planet. However, the evidence shows a more complex image, in which distinct lineages, such as neanderthals, denisovans, early H. sapiens, and other groups, interbred repeatedly during the late Pleistocene, in a process that aided the evolution of our genetic make-up.

Havarti and Ackermann’s Work

Recent scientific consensus has understood that the process of adaptation involves, not near-complete replacement, but partial replacement and a lot of reticulation. Havarti and Ackermann show that this process of hybridization between hominins left visible effects on the skull. Their work is the first to tackle the question of hybridization by studying the fossil records and different strands of evidence.

They investigated the fossil record of the late Pleistocene period in Western Eurasia, and compared it with recent genetic and comparative data, in order to see how skeletal data reflected admixture. Ancient fossil records were those of unadmixed as well as admixed groups. 

Three regions in the skull were examined: the mandible, braincase, and face, in order to determine if there was any hybridization. This would be detected through any intermediate morphology, dental abnormalities, or unusual sizes, all of which are present in hybrids of different mammals, such as primates. The scientists found that hybridization was visible in the mandible, and braincase, but not in the face. In other words, the preservation of hybridization signals occured differently in the various cranial regions. 

They also investigated whether hybridization signals were related to the proportion of Neanderthal DNA. They found that this was not the case, with the presence of genetic variants being more important than the share of Neanderthal DNA. 

Where they did not know the genetic makeup of the fossils they had, the duo identified potential hybrids from different regions in the world, such as the Middle East, and Europe, but these identifications were not done using genetic data, and so, remain speculative, and this hypothesis will need to be verified. 

The Importance of Hybridization

Hybridization has been recognized as an important part of evolution for some time, driving novelty and diversity. Around 10% of animals, such as bears, bovins, canids and cats, are the product of hybrids. Primates are also known to have hybrids, with baboons a notable example. The purpose of hybridization is to introduce new variants, create novel combinations, potentially speed up evolution, especially in periods of uncertainty and unique challenges. 

Hybridization among humans has been less studied, but it likely played a similar role, allowing hominins to better adapt to their changing environment, especially as humans spread out of and into Africa, in waves of migration, as an earlier paper by Lu Chen, Aaron B. Wolf, Wenqing Fu, Liming Li, and Joshua M. Akey posited. These migratory waves demanded that species evolve in diverse ways to make them more resilient, and hybridization was certainly an important driver of that genetic innovation.

Prior to Chen et al, the evidence suggested that Africans did not have neanderthal DNA. By finding evidence of neanderthal DNA in Africans, we have a more complete and nuanced picture of migration and evolution and the role of hybridization, in driving human genome variation in response to the environments that humans encountered in the past. 

This work serves to place hybridization in a more positive light. In the past, hybridization has been seen as an aberration and an evil. Ancient texts are replete with warnings about hybridization, and mythology is filled with terrifying hybrids of man with other animals. For instance, the Greek Chimera was a fire-breathing creature who had the head of a lion, a snake for a tail, and the head of a goat on its back. When the Salk Institute announced that it had the first human-pig hybrid, this was met with understandable horror. This kind of research, however, has enabled the possibility of growing human organs in animals in order to use them in transplants. Indeed, in many countries, there is now a moratorium on funding for hybridization projects of this kind. Nevertheless, the research shows that historically, there are gains to be made from hybridization, and so, medical advances may be possible within ethical guidelines.