Restless Humans

Early humans wandered restlessly whereas apes stayed put in trees.

Posted Apr 30, 2020

Early humans migrated widely through Eurasia. Why were they so restless and why did their great-ape ancestors remain in place?

Leaving the Trees

Fossil evidence suggests that human ancestors lived in mixed grassland and trees (savanna) by approximately four million years ago.

At this time, some hominins (i. e., humans and great apes) walked upright based upon the relatively complete remains of Lucy. Lucy had the hips, and knee, anatomy associated with bipedalism. Lucy is essentially a missing link between apes and humans, having some undeniably modern traits and some distinctly simian ones like smaller brains and larger intestines. She would have lived in trees, much as chimps do.

Moving around in trees is laborious and requires all four limbs. Some primates can swing by the tail that thus becomes a fifth limb. Moving in grassland is easier and may be accomplished in bipedal fashion. Nevertheless, bipedalism likely predated living in the savanna and our ancestors walked upright while still living in trees, possibly because this is an efficient method of moving along large branches.

This phenomenon is illustrated by many desert animals who have even less vegetation to contend with. Animals as diverse as kangaroos, and jerboas, get around by bounding on two legs.

Among primates, hamadryas baboons, that inhabit open grasslands, remain quadrupedal but often rear up on their hind legs to scan the horizon for possible threats from predators.

Ancestral hominins living in savannas were vulnerable to large predators such as lions and cheetahs. Upright posture would have enabled them to peer through tall grasses for signs of predator movements.

The theory that many features of modern humans originally emerged as adaptations to life in more open conditions is known as the savanna hypothesis.

The Savanna Hypothesis

Scholars are often ambiguous about what savanna means. For some, it means open grasslands dotted with occasional trees. For others, it refers to a more mixed distribution of trees and grass. In either case, savanna means that there are large areas devoid of trees.

One often hears that hominins left the trees but it may be more accurate to say that the trees left us! The emergence of drier conditions likely resulted in the gradual loss of trees creating open spaces that were colonized by grasses.

Hominins responded to the open areas differently from other great apes, apparently being more attracted by the wealth of game animals on grasslands than repelled by the numerous predators.

Most great apes are expert tree climbers, with gorillas, who spend much of their time on the ground, proving an exception possibly because they are large enough to intimidate any predator. Gorillas still sleep in tree nests as protection from predators,

Other apes feel safest in trees and avoid open areas where they could easily be picked off by ambush predators, such as lions and cheetahs.

Hominins braved the dangers of the savanna because these were balanced by significant rewards, specifically high-energy foods like meat. Some archaeologists believe that early humans got into the hunting way of life indirectly either by scavenging from the abandoned kills of large predators like lions, or by frightening hunters away from their kills and stealing the booty.

Homo Erectus Migrations

If hominins began life in the open some four million years ago, it would be another two million years before our direct ancestor, Homo erectus emerged in the fossil record. Before going extinct, erectus demonstrated many of the behavioral characteristics of modern humans.

They were recognizably human in terms of body proportion (with longer legs and shorter arms than apes), height, gait, and brain capacity of 850-1100 cc that overlaps with the distribution for modern humans. Homo erectus is probably an ancestor of anatomically modern humans.

They were skilled makers of stone tools (although their tool complexity did not vary over time probably because they lived in small groups that failed to propagate local improvements).

Most significantly, erectus were restless wanderers and evidence of their activities is found throughout Eurasia in addition to Africa. They evidently mastered navigation having visited otherwise inaccessible islands approximately one million years ago based on artifacts recovered there.

We do not know what their boats looked like but may have been similar to the dugout canoes fashioned by many indigenous people to this day.

Successful as erectus were in mastering the challenges of many new habitats, they probably did not have spoken language given the relative underdevelopment of the musculature of the tongue (as represented by lack of striation of the hyoid bone beneath the tongue). Scholars speculate that they must nevertheless have had a fairly sophisticated capacity for communication.

Their use of fire may date from a million years ago but the capacity to control fire and use it for cooking is only about 400,000 years old. This innovation had a profound impact on human evolution, probably making us more intelligent, and even more expansionary, as discussed in the second part of this post.