Climate changes have been driving the evolution of the human genus since our ancestors first ventured to walk upright, through the emergence of our species, Homo sapiens, and the great migrations of Huns, Mongols, and Vikings that changed history.
Climate is vitally linked to ecosystems, so any change forces living beings to adapt, move to a more favorable location, or become extinct. The human genus seems to have excelled at the first two options throughout its two million-year history.
The ability of species to adapt to environmental transformations is known as natural selection, one of the main engines of evolution, first described in Charles Darwin's book On the Origin of Species in 1859.
However, it wasn't until 1925 that Raymond Dart proposed that climate had been a significant driver in human evolution. This marked the beginning of the Savannah Hypothesis, which suggests that our ancestors took a different evolutionary path from other primates to adapt to life on the African savannas, which emerged due to global cooling between 5 and 10 million years ago.
Nearly a century later, this remains the leading hypothesis to explain why natural selection favored an adaptation like bipedalism, an evolutionary path that began around 6 to 7 million years ago and eventually led to modern Homo sapiens.
But both paleoanthropologists studying human evolution and paleoclimate researchers suspect that changes in temperature, rainfall, and vegetation have led the human genus to migrate as a form of adaptation, not only within its native Africa but also to other continents. This pattern would have made it a global migrant.
A Planetary Wobble
Understanding how the climate changed in the regions inhabited by our ancestors hundreds of thousands or millions of years ago is no easy task. Fossil evidence can reveal what vegetation was like at a given time and what types of animals lived in the area, but covering vast periods to understand how and when the different species of the human genus emerged is another matter entirely.
A recent study published in the scientific journal Nature by Axel Timmermann and colleagues has provided evidence that the migrations of our species may be linked to changes in the planet's temperature, and may even have contributed to the emergence of different species, including our own.
This is a colossal simulation that spans 2 million years of human history. It was carried out by one of the fastest computers on the planet known as Aleph, at the Pohang University of Science and Technology in South Korea.
Researchers first fed Aleph all the necessary data, such as information from 3,245 fossil remains scattered across Africa, Asia, and Europe from six human species: Homo habilis, H. erectus, H. ergaster, H. heidelbergensis, H. neanderthalensis, and H. sapiens.
They also fed it information about the Earth's astronomical movements. The gravitational pulls and pushes generate 100,000-year cycles in which the planet's orbit around the Sun alternates between a more circular shape, with longer summers, and a more elliptical one, with longer winters that generate glacial periods.
At the same time, the Earth is not perfectly upright in space, it has a tilt, leaving one hemisphere closer to the sun and the other farther away. This is what gives us the seasons. This tilt changes every 41,000 years, in what are called Milankovitch cycles, which completely alters rainfall patterns and, with them, the climate of each ecosystem on the planet.
Equipped with all this knowledge, Aleph then created a detailed climate model of the last 2 million years of the planet. This allowed scientists to recreate what the habitats and climate changes were like in each of the sites where the 3,245 human species fossils were found.
Having so much information and a supercomputer allowed them to cross-reference this data in any way they wanted. For example, by comparing the ecological and climatic niches of each human species, they were able to see that ours, Homo sapiens, preferred to live in regions with dry conditions, but could also do so in other areas.
Once the researchers were able to identify the type of habitat each human species preferred, they could see how they changed over 2 million years and how each species adapted to those changes. Whether they evolved, migrated, or became extinct.
The First Great Traveler
According to the data provided by paleogenetics, the study of the DNA of our ancestors, we are all immigrants. Every population on the planet descends from one that came from somewhere else. The first migrant was Homo erectus.
This human species is the first to have an anatomy very similar to ours, the first to have a varied diet that included meat, and is believed to be the first to master fire.
H. erectus already had a rudimentary stone technology that allowed them to exploit a wide variety of foods. This entire toolbox allowed them to become a global traveler.
The oldest remains of H. erectus are from the Omo site in Ethiopia, dating back 2.4 million years. From there, this ancestor of ours spread to various parts of East and North Africa. By 1.77 million years ago, it was already present in Dmanisi, Georgia, on the border between Europe and Western Asia. Just 100,000 years later, it was also found in China and Southeast Asia.
H. erectus's transition to global traveler was possible because they could adjust their behavior to a diversity of ecological situations. They were already omnivores, capable of eating a wide variety of foods, making them flexible generalists.
As Timmermann and colleagues were able to verify in their Nature study: "H. erectus had already begun the transition from regional residents to global travelers before 1.8 million years ago. This indicates that the dispersion from Africa always involved an adaptive change, whether biological or cultural, to more varied climates."
Although they remained in certain types of ecosystems and migrated in search of them, it would be their descendants who would become fully global travelers.
Mother of Sapiens and Neanderthals
Paleoanthropology, the study of our ancestors' fossils, and paleogenetics have a clear favorite as our most direct ancestor: the species Homo heidelbergensis. It is believed to have evolved in Africa from H. erectus about 800,000 years ago.
Its fossil remains have been found in Africa, Asia, and Europe. In the latter continent, it was also the mother species of another close relative, the Neanderthals.
The results of the supercomputer Aleph simulation support the idea that H. heidelbergensis was a global traveler with a greater adaptive capacity than its ancestor H. erectus. "It began expanding its habitat about 700,000 years ago," Timmermann and colleagues say in the study.
The model suggests that the distribution of H. heidelbergensis across different continents was possible thanks to the Earth being in a more elliptical orbit at that time. This created conditions for warmer climates, which expanded the preferred habitats of this human species, in turn enabling more spaces to migrate to.
Interestingly, the simulation performed by the supercomputer Aleph shows that fossils and archaeological remains are not distributed randomly, according to how and where they are discovered. The regions that Aleph's simulation classifies as most habitable coincide with those with the greatest abundance of remains, and conversely, the least habitable areas are where there are very few or no fossil and archaeological evidences.
The simulation also shows that about 400,000 years ago, there were two periods of climatic stress in southern Africa, which were unusually warm. This led to H. heidelbergensis losing habitable spaces. It is a time for which there are almost no fossils of this species, and about 300,000 years ago, those of our own species, Homo sapiens, begin to appear.
This would be consistent with the hypothesis of some paleoanthropologists, based on fossil and ancient DNA evidence, who say that H. heidelbergensis evolved into us in the southern African region in a single evolutionary event. Some group could have adapted to these warmer and drier conditions, evolving into us.
Precisely, the study also indicates that all the fossil and archaeological remains of prehistoric Homo sapiens coincide with an expansion of their habitats to include dry climates. Our species experienced a period of climatic stress about 210,000 years ago, from which it emerged successfully, adapting to an even greater variety of climates, which in turn provided it with more genetic diversity, that is, variety.
These climatic stress events are key to generating natural selection, and thus the evolution towards new species. This is what also happened with Homo heidelbergensis in Europe, about 400,000 years ago, due to changes in the Earth's orbit that generated the Ice Ages.
As a product of this stress, evolution facilitated the appearance of a new species better adapted to this type of climate, and thus, gradually, between 400 and 300 thousand years ago, H. heidelbergensis evolved into Neanderthals, or Homo neanderthalensis.
The data provided by the Aleph simulation coincide with those obtained by analyzing fossils and the DNA of these prehistoric human species. Thus, it is also seen how our species began to coincide with Neanderthals in the spaces they occupied in Eurasia. Eventually, Neanderthals disappear from the map, also due to climate changes and their crossing with sapiens. Although they continue within us, with their genes scattered throughout the DNA of current humans.
The Great Migrant
It's no coincidence that we replaced other species on the continents we reached. Homo sapiens' greater tolerance to different climates greatly expanded its ability to move. This facilitated multiple waves of dispersion within Africa, between 200 and 100 thousand years ago, and into Eurasia, crossing the Sinai and the Arabian Peninsula, about 100 to 60 thousand years ago.
But it didn't stop there. Our species has become the quintessential migrant. About 50 thousand years ago, it already inhabited the islands of the Pacific and Australia, and about 30 thousand years ago, it began to populate America.
But did it stop after populating all the continents? No. Migrations never stopped, as paleogenetic studies clearly show.
These studies tell a fascinating story: the current European populations, for example, are the product of dozens of major migrations from Asia, also motivated by climate changes.
"Ancient genomes allow us to reconstruct migrations and population divisions that can be dated using purely genetic tools, based on diversity and mutation rates that are now widely accepted, after more than twenty years of the Human Genome Project," explains Carles Lalueza Fox to Muy Interesante, a paleogenetics expert and researcher at the Institute of Evolutionary Biology of the Spanish National Research Council and director of the Barcelona Museum of Natural Sciences.
"These data can be correlated with demographic models and aspects of 'machine learning'," continues Lalueza Fox. "The fact that these divergence dates can be correlated with paleoclimatic data is very interesting and undoubtedly points the way towards a multidisciplinary vision of human evolution, which will soon also be able to be correlated with paleoproteomic data."
"It would seem that both genetic and paleoclimatic data continue to point to an origin of our species located on the African continent," says the expert. "I think it would be interesting in the future to explore the possibility of climatic environments that favored long pan-Eurasian longitudinal movements, both in the Middle Pleistocene and the Upper Paleolithic, where large-scale population movements and replacements also continue to occur."
These are the great migrations that Lalueza Fox talks about in his recent book Inequality, in which he shows how these large movements can be seen by analyzing the genes of current European and Asian populations, comparing them with the DNA extracted from bone remains of burials that cover the last 10,000 years.
This allows us to tell a story in which the first farming peoples of what is now Turkey, about 7,000 years ago, completely replaced the hunter-gatherers who inhabited Europe. There were several more of these total replacement migrations.
They were followed by the historical ones of the Huns, the Germans, the Mongols, and the Vikings. All great movements of peoples motivated by climate changes, which were key to history and in shaping the current peoples, even those who have come to believe themselves "pure".
Although we have always known that humans are inveterate travelers, the results of the supercomputer Aleph simulation imply that, during the 2 million years of human history, climate changes played a central role not only in the emergence of new species but also in shaping the current cultures of every corner of the planet.
Climate change has been the engine of the evolution of life on Earth, as we have seen, although humans have shown that they can overcome the obstacle that means everything changing around them. But now, that ability, human activity, is driving thousands of species to extinction by accelerating climate change and preventing them from adapting.
Comments
Post a Comment