Humanity’s ancestors almost disappeared – 02/09/2023 – Reinaldo José Lopes

Certain ideas, as the Witch of Cosme Velho used to say, deserve to be instilled in the soul by force of repetition. Here is one of them, which every now and then appears in this column. It is very easy to forget how the overwhelming presence of human beings as the dominant species of this third little planet of the Sun’s domain is a recent and fragile phenomenon, at least from the point of view of geological and cosmic time scales. Let’s do some math first.

The lineage of great apes that led to the homo sapiens it probably split off from the branch of the Tree of Life that would give rise to today’s chimpanzees, our closest living relatives, about 7 million years ago. Our species is much more recent, with an age that reaches, at most, something like 300,000 years.

For practically all that time, we never stopped being, deep down, just another species of large mammal. During the end of the Ice Age, we gained a geographic distribution of worldwide dimensions, it is true, but this was not very different from what other cosmopolitan predators had already achieved, such as wolves and lions (both present in almost all regions of the Old and New World before we cornered them).

That is, it is quite possible that there were not much more than 1 million people in the entire world until a mere 10,000 years ago. It was only from about ten millennia before present (the proverbial blink of an eye, from Earth’s point of view, and even from our separate evolutionary history) that the process of population growth and increasing technological complexity began, bit by bit. that made us into the geological force we are today.

The reasons for this almost miraculous success are manifold, although one of the most important is certainly the domestication of plants and animals. What we don’t usually take into consideration, however, is that, before humanity’s explosive advance began, we suffered from a fragility rigorously identical to that of other large mammals in the face of the ups and downs of evolution. And if a new study is correct, our ancestors hit the brink of extinction even before the homo sapiens existed.

Imagine the situation: a world in which only about a thousand individuals of this species of ancient humans were reproductively viable. In essence, this is what a paper that has just been published in the specialized journal Science, one of the largest in the world, says.

Coordinated by Haipeng Li, from the University of the Chinese Academy of Sciences, the study analyzed the DNA of people alive today to estimate the genetic diversity of humanity’s ancestors along our evolutionary path. You can do this because there is a correlation between DNA variations and population: the more people reproduce, the greater the chance of mutations appearing in the DNA that multiplies, roughly speaking.

Haipeng and his colleagues estimate that 900,000 years ago there was a drastic population reduction (perhaps on the order of 90%) in the African lineage of the genus Homo (ours) which later gave rise to the H. sapiens and to our closest extinct cousins, the Neanderthals and Denisovans. For tens of thousands of years, we would have occupied the uncomfortable position of a modern-day blue whale or golden lion tamarin.

The idea would help explain the relative lack of hominin fossils (our lineage) at that time. If the proposal is correct, it is one more indication of how contingent our climb towards planetary dominion was. It is convenient to see it with much less arrogance than the one we have so far.

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