Scientists are proposing a new theory of human evolution.
A groundbreaking new analysis of data suggests that key evolutionary changes in prehistory were driven by cyclical changes in tropical climate.
In the past, most scientific attention had been paid to northern hemisphere ice ages as an evolutionary driver – but now scientists are suggesting that changing cycles of El Niño and La Niña weather phenomena, in a zone around the equator, were also key contributors to human evolution over the past 3.5 million years.
Ice ages and tropical climate cycles are both caused by a common non-climatic driver – cyclical changes in Earth’s solar orbit and in the way in which our planet spins through space.
To demonstrate a correlation between key human evolutionary developments and tropical climatic changes, scientists have been carrying out an unprecedented analysis of climatic data obtained from lake sediment and seabed cores extracted from Africa and from the seas and oceans around it.
So far, researchers from Germany, the UK, the Netherlands, the USA and Ethiopia have examined data from lake sediments in Kenya, Malawi, Ethiopia and Ghana and seabed sediments from off the coasts of Tanzania, Namibia, Congo, Mauritania, Libya and Egypt.
The analysis of those cores, led by palaeoclimatologist Dr Stefanie Kaboth-Bahr of the University of Potsdam, has now allowed the scientists to prove a correlation between human evolution and tropical climate changes over the past 600,000 years.
Now they are planning to extract more cores in order to push scientific knowledge of the African climate record back a further 3 million years.
Because Earth’s orbital and rotational changes over that period are already known, the scientists already know theoretically what the cores are likely to reveal.
Their paper, published today in the US science journal PNAS, demonstrates a correlation between a big change in tropical climate and the emergence of our ancestral species (archaic Homo sapiens) about 300,000 years ago.
But other, very major changes in tropical African climate almost certainly also took place about 2.4 million years ago, 2 million years ago and 800,000 years ago – and each of those probable changes correlates with key chapters in humanity’s evolutionary story.
It is believed that each of the tropical climatic changes led to abrupt reversals in which parts of Africa were either more or less easily habitable.
The ongoing research suggests that, in some periods, west Africa was bone-dry and east Africa was lush and humid – but in other periods, the exact opposite was the case.
It is likely that large-scale rapid reversals occurred at least eight times over the past 3.5 million years – and that, as a result, early humans had to migrate and rapidly adapt multiple times.
The scientists are now proposing that it is those climate reversal-induced adaptive pressures which led to greater migration flows, increased gene flow, faster environmental adaption, accelerated evolution and the emergence of new human species.
Among the key ancient human species which may have come into existence fully or partly because of tropical climate reversals were the first humans to make sophisticated stone tools (Homo habilis) 2.4 million years ago, Homo erectus (at least 80 per cent smarter than its predecessors) 2 million years ago, Homo heidelbergensis 800,000 years ago and our own immediate ancestor species, archaic Homo sapiens, 300,000 years ago.
Climate scientists believe that each crucial tropical climate reversal event began not in or around Africa, but in our planet’s largest ocean, the Pacific. Changes in Earth’s orbit and rotation cause changes in the amount of solar heat impacting the Pacific and that in turn leads to periodic atmospheric pressure changes in the air above the ocean.
Those atmospheric changes then spread into the Indian Ocean, Africa and finally into the Atlantic, thus triggering tropical climatic reversals in the African continent.
“We are proposing a completely new climatic framework that can be used to investigate correlations between tropical climate and archaeological and fossil human material over long time periods,” said Dr Kaboth-Bahr.
“The data so far suggests that we need to more fully understand tropical climate changes in order to better understand human evolution,” she said.
Another key author of today’s PNAS paper and a leading authority on the relationship between climate and human evolution, Professor Mark Maslin of University College London, said the continuing research had the capacity to fundamentally change the world’s perception of the mechanisms driving human evolution.
“The understanding of the relationship between rapid climatic changes in Africa and the environment in that continent is critical if we are to understand the fundamental causes of the major elements of human evolution,” said Professor Maslin, author of a key recent book on human evolution in Africa called The Cradle of Humanity.