There is a new theory on where our species, Homo sapiens, came from, and it was recently published in Nature (Ragsdale, A. P. et al., “A weakly structured stem for human origins in Africa,” Nature [2023]).
According to the currently accepted idea, Homo sapiens emerged from a small group of people living in Africa between 300,000 and 100,000 years ago. This scenario postulates that the new species quickly became dominant, displacing the older Homo species.
However, more detailed tracking of the evolution of modern humans has been difficult due to the tiny number of human fossils known from Africa and the lack of ancient DNA during that period.
The new theory, supported by extensive genetic research on modern populations, suggests that Homo sapiens evolved from merging multiple, geographically distinct people that maintained close enough ties for gene flow to cause their evolution to occur roughly simultaneously despite superficial differences in appearance.
Instead of the traditional “tree of life” model, in which local populations branch off and become genetically isolated, giving rise to new species, the authors of the new study characterize human evolution as a “weakly structured stem” that more closely resembles a tangled vine, consisting of multiple, interacting regional populations.
This new model has important implications for understanding how human evolution was based on a complex dialectic between culture and biology, as opposed to the more purely biological mechanisms of natural selection which govern the development of other species. However, this is not explicitly called out in the Nature article.
This tangled vine of an idea is similar to the “single-species hypothesis,” which has been unpopular for decades but is seeing a resurgence of interest in recent years. Statistical study of a massive trove of genetic data culled mainly from African and some Eurasian populations, as well as Neanderthal fossils, supports this alternative theory.
To better trace migrations and interactions between groups over the previous 150,000 years, this study models genetic diversity between modern populations and projects it back in time. This finding reveals substantial interregional gene exchange throughout time.
This study lends credence to the theory that the superficial morphological differences currently observed amongst groups result from a continuing population shuffle that has been going on for hundreds of thousands of years.
While it has been widely accepted that H. sapiens evolved in a single population before spreading outside, the authors argue that this idea does not square with the available archaeological or fossil evidence. Physical evidence points to an overall appearance of modern human artefacts and fossils across Africa simultaneously (on a geological time frame) rather than a single origin and subsequent spread.
The current model, which postulates a single local point of origin, has gained traction because it fits with the overarching trend in biological evolution, according to which geographically separated populations of the same species tend to be genetically isolated, with minimal gene flow between them.
The accumulation of random mutations, which tend to vary from population to population, is known as “genetic drift,” it is aided by the lack of gene flow. This interacts with distinct forms of selective pressure from regionally distinct ecosystems. Together, genetic disparities tend to increase over time, reaching a tipping point where people from different populations are genetically incompatible and can no longer have healthy offspring.
This process is called “speciation.” The mule illustrates this process well in its intermediate but not final form. Mules, the offspring of horse and donkey matings, are healthy and able to live independently, but they are usually unable to reproduce.
This means that the two parent species cannot exchange genetic material effectively. However, coydogs have fertile offspring from matings between coyotes and dogs. That’s why you can’t call them two different species.
An opinion (not expressly addressed in the Nature article) that the genus Homo, at least in its latter stage of development, does not correspond with the mainstream speciation model is supported by the newly suggested hypothesis of modern human beginnings.
For the most part, humans have relied on cultural shifts to adapt to new environments instead of altering their physical appearance. On the other hand, culture is more malleable and dynamic than biology and may spread rapidly and efficiently from one community to another.
Through abstract thought, humans may analyze the aspects of unknown settings and generate suitable adaptations to tools, techniques, and social organization patterns. Because of this, humans have effectively colonised a wide variety of habitats, from the Arctic to tropical rainforests and deserts, with little to no physical adaptation and, thus, without speciation.
New evidence suggests that not all early Homo populations, even those in far-flung regions (such as Homo naledi and the Flores “hobbits”), exhibited the degree of change resulting in speciation, despite showing some genetic variation. One prime example is the discovery that approximately 2% of modern Eurasian Homo sapiens carry Neanderthal DNA.
This proves that these populations were not genetically isolated despite their morphological differences and interbred successfully when modern humans migrated out of Africa. As a result, they did not stand for separate species.
That modern humans are the result of a complex, dialectical interaction between physical and cultural adaptation to a degree qualitatively distinct from all other animals is supported by the proposed “tangled vine” model of Homo sapiens evolution, which is not only more in accord with the available archaeological and biological evidence. You can follow us on Twitter if you are interested in reading more recent news about the Origins of Homo Sapiens, and you can do so by clicking here.