Erratic development: The genetic basis for our humanity developed in several temporal thrusts, as revealed by comparative DNA analyses. According to this, many gene variants that are decisive for Homo sapiens arose more than 300,000 years ago. Then, around 50,000 years ago, there was another surge in genetic innovation. It primarily affected the brain and behavior and coincided with the spread of Homo sapiens in Eurasia.
At the beginning of human history there were still many different pre-human and early human species. But only Homo sapiens prevailed and became the only human species on our planet today. But why? What made our ancestors so successful and what made them different from their predecessors and contemporaries? So far, this question has only been partially clarified and the genetic basis for many “typically human” characteristics is also partly in the dark.
Mosaic instead of linear development
What seems clear, however, is that, contrary to what was long thought, human evolution did not proceed in a straight line and in a single strand. “The anatomical features that characterize us as a species did not appear in one package and in one geographical location, but developed gradually and in a mosaic that encompassed the entire African continent,” explain Alejandro Andirko of the University of Barcelona and his colleagues. For example, fossils show that various early humans already developed parts of the modern human anatomy.
Behavioral traits and mental abilities previously thought to be unique to Homo sapiens have also been identified by anthropologists in close relatives of our ancestors, such as Neanderthals. These include rock art, jewelery and burial rituals. “The range of early human diversity surprised anthropologists,” says Andirko. Conversely, some early Homo sapiens fossils still showed archaic features.
Human-typical gene variants traced
But what about the genetic evolution of Homo sapiens? When did the genes that make us human and distinguish us from all other hominids evolve? Andirko and his team have now investigated this using genome comparisons. To do this, they evaluated databases of human gene variants that record more than 4.4 million point mutations in the genome of people living around the world today.
From these gene variants, the researchers primarily selected so-called high-frequency gene variants for their analysis. These regions of the genome are characterized by mutations that only occur frequently in Homo sapiens. Through comparative analyzes and with the help of a special algorithm, Andirko and his team determined when these gene variants appeared and how this is related to the chronological cornerstones of Homo sapiens evolution.
Two clear peaks of genetic innovation
The result: Our ancestors experienced two distinct episodes of genetic change – one more than 300,000 years ago and a second between 90,000 and 40,000 years ago. “This distribution in the occurrence of high-frequency gene variants corresponds to two periods of great importance for the evolutionary history of Homo sapiens,” the scientists explain. “We were able to clearly identify different epochs and the variants associated with them.”
Through more detailed analysis, the team was able to determine which functions and organs they influence for some of these gene variants. In addition to bones, muscles and other anatomical features, this also includes hormone metabolism and the brain. “We found that the brain tissue in particular differed in their expression profile at different times in our history,” reports Andirko. “Certain genes of neuronal development were therefore more active at some times than at others.”
From early humans to Homo sapiens
The first peak of new genetic variants was a good 300,000 years ago and thus at the time when Homo sapiens was just separating from other members of the Homo genus. At that time, our ancestors developed, among other things, the straight face without a protruding jaw and other anatomical features of modern humans. This is evidenced by the fossils of the oldest Homo sapiens discovered to date in Jebel Irhoud, Morocco, in 2017.
At the same time, the gene variants that emerged at this time could have given the nascent Homo sapiens new mental abilities and behaviors. Andirko and his team found evidence of this in some gene variants from this period that promote the networking of the brain. The newly acquired skills may have enabled our ancestors to deal better with environmental changes.
departure into the world
The second phase of a particularly large number of new gene variants took place around 90,000 to 40,000 years ago. This corresponds to the phase in which Homo sapiens left Africa and began to colonize other continents. At that time, our ancestors mainly developed some new mental and coordination skills, as the functional analyzes of the gene variants show. Some mutations that occurred around 50,000 years ago caused the corpus callosum, which connects the two halves of the brain, to become significantly larger.
Gene variants that increase the volume of gray matter in the cerebellum also emerged around the same time. Contrary to what was long thought, the cerebellum is not only responsible for movement control and coordination, but also plays an important role in many higher brain functions – from attention to decision-making.
“The” human gene does not exist
According to the research team, their results show how and when genetic innovations shaped the evolution of Homo sapiens. At the same time, however, they also illustrate that the emergence of modern humans was not the result of just one or a few genes. “We found no evidence of evolutionary changes based on just one or a few key mutations,” says Andirko.
Instead, many tiny changes and their accumulation at certain times made us humans who we are today. (Scientific Reports, 2022; doi: 10.1038/s41598-022-13589-0)
Source: University of Barcelona