Why Did Our Ancestors Leave the Forests While Apes Stayed? — New Theory Suggests This Is How It Sparked Our Evolution

Humans and apes shared a common ancestor that lived a few million years ago. But what happened during all that time, for our species to diverge and go on their separate evolutionary paths? One of the leading and new theories that answers this question is the open savanna hypothesis.

It states that our early ancestors lived in forest environments, much like today’s apes; however, about 4–7 million years ago, many hominin species began leaving the area for open savannas. This move led to the evolution of several traits and characteristics that make humans unique from apes. So, why did our ancestors leave the forests while apes stayed?

From Forests to Fields: A Major Shift in Human Evolution

The move of modern humans’ ancestors from forests to open savannas was driven by ecological changes. It is not as simple as the common ancestor venturing out of the forest to explore or find food. The open savanna hypothesis also states that the forests receded and open savannas expanded.

This environmental change, triggered by climate and glacial cycles that early humans were not totally in control of, forced them to open or semi-open grasslands. This is where survival favored hominins to stand upright to check for predators, walk bipedal to be able to use their arms and hands, and shed their fur to thermoregulate as savannas require long-distance travel under the heat of the scorching sun.

In other words, the expanding grasslands created an evolutionary pressure for humans to change and adapt their anatomy, physiology, and eventually behavior, starting the divergence of hominins from their tree-climbing ape relatives.

Read morePubMeded

Why Apes Stayed Behind

Evolution from ape to man. From Proconsul to Homo heidelbergensis

Unlike hominins that were forced to open savannas or grasslands, apes’ ancestors stayed in the forest areas. As those regions receded and savannas expanded, only a limited number of species were able to survive. Early humans either adapted and evolved or perished.

But for those who were able to stay in the forests, such as the apes’ ancestors, evolution favored stronger upper bodies for climbing and reaching the fruits in the trees. They did not need to walk upright or develop bipedalism.

This split in lifestyle in both species led them to change in order to hone their survival strategies, and eventually their own bodies and genomes. Because when there is a divergence in environment or lifestyle, natural selection favors the changes that will bring the highest probability of success in the new environments or circumstances.

In this case, success means both survival and reproduction. These events could lead to the development of a new species coming from separate evolutionary paths.

The Role of Bipedalism

One of the most significant changes in our evolutionary path that most people often overlook is the freeing of our hands. While apes still use their arms to grab tree branches when reaching for fruits, humans began to develop bipedalism and stand upright.

The savanna hypothesis states that bipedalism freed up our hands for carrying food and tools. Additionally, upright posture also allowed our ancestors to look over tall grass, and cooperative hunting began to catch the faster and bigger animals.

There are fossils like Lucy or Australopithecus afarensis that show how the early adaptations of transition to upright walking back in 3 to 4 million years look. This is right around when the savanna environments began expanding across Africa, and when early humans began our significant divergent evolutionary path from apes.

Read more: Wikipedia

Evolution Sparked by Open Landscapes

Living in the savanna not only changes our locomotion, but it also influences our brains and way of life. Due to how hard it was to take down other animals in the open savannas because of their raw speed and size, hominins cooperated with each other to hunt successfully.

Scientists think that this hunting cooperation and the formation of larger groups led to the development of our brain. To navigate complex social structures, particularly during hunting, early humans were required to have enhanced cognitive abilities for coordination, communication, and communal problem-solving.

However, in apes’ ancestors’ case, they could find food by simply climbing the trees in the forest areas, and they usually needed a social structure for protection against other predators and increased access to resources.

The Seven-Million-Year-Long Journey - Why Humans Stood Upright ~ with JEREMY DESILVA

Author's Final Thoughts

The savanna hypothesis remains one of the most compelling explanations for how humans diverged from apes and evolved to be who they are today. Moving from forests to grasslands, driven by climate shifts or expanding savannas, shaped our bodies, minds, social structures, and ultimately, our future as a species.

Read next: Evidence Suggests Modern Humans First Left Africa at Least 100,000 Years Ago — But This Is Why That Initial Migration Failed

References & Further Reading

Potts, R. (2012). Evolution and environmental change in early human prehistory. Annual Review of Anthropology. https://doi.org/10.1146/annurev-anthro-092611-145754

Maslin, M. A., et al. (2014). East African climate pulses and early human evolution. Quaternary Science Reviews. https://discovery.ucl.ac.uk/1447999/1/1-s2.0-S0277379114002418-main.pdf

Uno, K. T., et al. (2016). Neogene biomarker record of vegetation change in eastern Africa. PNAS. https://www.pnas.org/doi/10.1073/pnas.1521267113

Su, D. F., et al. (2024). Early Hominin Paleoenvironments and Habitat Heterogeneity. Annual Review of Anthropology. https://www.annualreviews.org/content/journals/10.1146/annurev-anthro-041222-102712

Cerling, T. E., et al. (2011). Late Miocene to Pliocene carbon isotope record of vegetation change in eastern Africa. PNAS. https://www.pnas.org/doi/10.1073/pnas.1018435108