Asked and Answered: Why Haven’t All Primates Evolved Into Humans?

Portrait photo of vervet monkey
For insight into primate evolution, UC Davis anthropologist Lynne Isbell and colleagues tracked vervet monkeys as well as olive baboons and leopards. Their findings offer new insights into the survival strategies of different primate species, including the ancestors of humans. (Lynne Isbell/UC Davis)

Many people mistakenly think of evolution as progress. But humans, despite their ability to manipulate objects and change their environment, are not “on top” of the world’s species. “In terms of success, what we’re talking about is how well organisms reproduce,” said Lynne Isbell, chair of the UC Davis Department of Anthropology.

Isbell recently shared her expertise in primate behavior with LiveScience for a story on why all primates haven’t evolved into humans. We followed up with Isbell to learn about the research behind her explanation in the article that humans aren’t “more evolved” than our closest cousins, chimpanzees.

“Evolution isn't a progression,” she told LiveScience. “It’s about how well organisms fit into their current environments.”

To each their own niche

UC Davis professor Lynne Isbell holding the head of a tranquilized leopard
With the help of a wildlife veterinarian and a tracker, Lynne Isbell and colleagues tranquilized and put GPS collars on four leopards to study their encounters with vervet monkeys and baboons in Kenya. The collars were later removed. (Courtesy photo) 

Isbell said the fact that humans became bipedal and migrated around the planet — while chimpanzees remained in Africa where they swing through the trees and knuckle walk on the ground — illustrates differing survival strategies rather than superiority.

In 1996, Isbell and Truman Young, also of UC Davis, suggested that as the climate changed and food became more spread out in African tropical forests, the ancestors of humans left the trees and began walking on two feet to cover ground more efficiently, which allowed them to remain in cohesive groups and eventually expand into savannas. The ancestors of chimps evolved a different survival strategy to deal with changing food conditions. They stayed on all fours and remained in the forest, but became more flexible in their group size, another way to reduce the energetic cost of travel.

A recent field study by Isbell and colleagues in Kenya suggests that the hominin strategy, while providing a unique solution to reduced food resources, came with added risks from predation once hominins left the trees.

Many scholars of primate evolution infer that ancestral humans climbed into trees or up cliffs to avoid ancestral leopards and other carnivores. “Most of the models out there assume that hominins would have been at greatest risk during the day,” Isbell said.

Instead, Isbell thinks our ancestors were most vulnerable at night to predators and so used stone tools to cut acacia tree branches and build thorny shelters to sleep within.

Masaai village in Africa
A boma, made from thorny acacia tree limbs, protect Maasai homes in Kenya. (Lynne Isbell/UC Davis)

Tools used to cut wood have been identified at Kenyan sites dating to 2 million years ago. Particles of acacia wood have been identified on 1.5 million-year-old handaxes in Tanzania. acacia shelters called bomas are used today to protect livestock in East Africa.

Tracking leopards and primates in Kenya

For more insight into primate evolution, Isbell and colleagues have studied predator-prey relationships in modern primates. They recently concluded a 14-month study in Kenya tracking leopards and two primate species that leopards hunt — vervet monkeys and olive baboons.

A mother baboon with baby grooms another adult.
Olive baboons can fend off leopards and other predators during the day, but are more vulnerable at night, Isbell found. She suggests that human ancestors, facing similar risks when they left the trees, learned to build thorny enclosures to protect themselves. (Lynne Isbell/UC Davis)

The research team put GPS collars on four leopards, 12 vervets in five groups and six baboons in four groups, and also used camera traps in order to unobtrusively study their movements and encounters.

Among their findings: vervet monkeys were more vulnerable to leopards during the day, while baboons were more at risk at night. But neither primate species was more vulnerable than the other.

“What we found in our study is that leopards don’t really bother the baboons during the day,” Isbell said. “They kind of hide because baboons can kill them. But at night, the [sleeping] baboons were at greater risk.”

“We couldn’t help but draw inferences to early hominins, who, like baboons, would have been able to defend themselves during the day but would have been highly vulnerable at night,” she said.

The president-elect of the American Society of Primatologists and an associate editor of both the Journal of Human Evolution and the American Journal of Physical Anthropology, Isbell earned her doctorate in animal behavior from UC Davis in 1990. Her research focuses on the socioecology of monkeys and apes, examining the ecological conditions that have influenced their social behavior and social organization. She is the author of the prize-winning book, The Fruit, the Tree, and the Serpent: Why We See So Well.

— Kathleen Holder, content strategist in the UC Davis College of Letters and Science