How did Pleistocene hominins cross the sea?

CENIEH researchers have participated in a paper published in the journal PLOS ONE presenting a computational model showing that hominins could have crossed the sea at certain straits over one million years ago

Ana Mateos and Jesús Rodríguez, scientists at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), are part of a team that has published a computational model today in the journal PLOS ONE, showing how hominins could have crossed the sea at certain straits more than one million years ago.

Hominins left Africa for Eurasia several times between one and two million years ago. Whether they dispersed exclusively by land or were able to cross certain straits, like Gibraltar in the Mediterranean Sea or Bab-al-Mandab in the Red Sea, is a point debated by scientists.

Other authors had already used computational models to study these dispersions, but they had always considered the sea as an insurmountable barrier. The model now published by Spanish and German scientists simulates the behavior of a hominin group in a hypothetical landscape with two banks separated by a strait, and assesses whether crossing and establishing a stable population on the other side could be successful.

To elaborate this model, the actions and interactions of individuals within an environment have been reproduced, including both landscape constraints (resources and topography) and physiological and demographic factors, and the hominins' decision-taking capacity. The efficacy of four kinds of movement over water were also evaluated: two of them active and directed (swimming and using a raft), and two others passive and undirected (drifting with and without an object to serve as a float).

“We also simulated the effect of the main physiological risks the sea could pose to a hominin in the Pleistocene, such as dehydration, hypothermia and exhaustion,” says Mateos.

Less than 10 km

The results show that the probability of success falls as the distance between the banks rises. These hominins could have crossed distances up to 10 km merely by swimming. The principal limiting factor would be the water temperature, which could cause death by hypothermia, even in warm seas like the Mediterranean.

“The use of simple rafts would allow longer crossings, but in these cases the main risks to be taken into account would be dehydration and starvation. However, it is likely that this technology was developed much later,” comments Mateos.

Taking decisions

Incorporating a certain decision-taking capacity for individuals into the model, even at a very basic level, has proved itself a highly useful idea. The results predict that the directionality of the movement, the perception of the opposite side and the intention to reach it are key factors.

In addition, as Rodriguez points out, “The probability of crossing a strait and successfully establishing a founder population on the other side by accident is very low, as this requires several individuals to pass at the same time."

Apart from the CENIEH, the other participants in this paper, whose lead author is Ericson Hölzchen, are scientists from Goethe University in Frankfurt, the Trier Lab for Social Simulation (TRILABS) and the German Research Center for Artificial Intelligence (DFKI).