Our earliest peregrinations may have started in the treetops.
I believe that we are drawn to certain terrains. We know instinctively what landscape feels most like home when we want to wander, whether it’s mountains, seascapes or wide, open ranges.
Our inclination for peregrination is old. In fact, the oldest unequivocal evidence of upright walking in the human lineage is bipedal trackways that date to 3.7 million years ago and that were discovered in Tanzania. Recently, however, footprints that predate them—originally thought to have been made by a bear—have just been verified.
But what if where we first started to “branch out,” so to speak, and traverse topographies on two feet was high in the trees? For decades it was assumed that bipedalism arose because we came down from the trees and needed to walk across open savannas. In the quest to understand how and why early humans started walking on two legs, scientists are now looking to modern chimpanzees living in dry, wide, savanna-like environments for clues. It may be that bipedalism didn’t evolve purely as a response to ground-based travel, but also for efficient and safe movement within trees.
In an effort to understand how and why early humans started walking on two legs, scientists are looking at modern chimpanzees who live in dry, open, savanna-like environments.
In another terrain-related study, it was discovered that more mammals were living on the ground several million years before the mass extinction event that wiped out the dinosaurs, meaning that landscapes were far more important for the course of Cretaceous mammalian evolution than any influence from the ancient archosaurs.
Our early walks on land
In 1978, renowned paleontologist Mary Leakey and her team discovered the oldest, unequivocal evidence of upright walking in the human lineage: footprints in Laetoli, Tanzania. The bipedal trackways date to 3.7 million years ago. In 1976, another set of footprints nearby had been partially excavated at a location called Site A, but they were dismissed as possibly being made by a bear. However, a recent re-excavation of those footprints and a detailed comparative analysis revealed that they were, indeed, made by an early human.
Laetoli is famous for its impressive hominin footprints at Sites G and S, which are generally accepted as Australopithecus afarensis, the species of the famous partial skeleton named “Lucy.” Because the footprints at Site A were so different, some researchers thought they were most probably made by a young bear walking upright on its hind legs.
Bears stand and walk on two legs for intimidation displays or to improve sight or smell. Some tracks made in Laetoli, Tanzania—a place famous for its hominin footprints—were so different that they were thought to have been made by a young bear walking upright on its hind legs.
To determine the maker of the Site A footprints, an international research team led by scientists at the University of Colorado Denver went to Laetoli in June 2019, where they re-excavated and fully cleaned the five, consecutive footprints. They then compared the Laetoli Site A tracks to the footprints of black bears (Ursus americanus), chimpanzees (Pan troglodytes) and humans (Homo sapiens).
The researchers then consulted with the Kilham Bear Center, a rescue and rehabilitation center for black bears in Lyme, New Hampshire. They identified four, semiwild, juvenile black bears at the center with feet similar in size to those who made the Site A footprints. The bears were lured with applesauce or maple syrup to stand up and walk on their two hind legs across a trackway filled with mud to capture their footprints. Over 50 hours of video on wild black bears was also obtained. It was found that the bears walked on two feet less than 1% of the total observation time, making it unlikely that a bear made the footprints at Laetoli, especially given that no footprints were found of this individual walking on four legs.
As bears walk, they take very wide steps, wobbling back and forth. They’re unable to walk with a gait like that displayed at Site A, state the scientists, as their hip musculature and knee shape does not permit that kind of balance and motion. Bear heels taper, and their toes and feet are fanlike; while early human feet are squared off and have a prominent big toe. Curiously, though, the Site A footprints record a hominin crossing one leg over the other as it walked, a gait called “cross-stepping.” Although humans don’t typically cross-step, this motion can occur when trying to reestablish balance. Scientists think the Site A footprints may have been the result of a hominin walking across an uneven surface.
Black bear toes and feet are fanlike (as shown in the footprint here), while early human feet are squared off and have a prominent big toe. Researchers recently investigated the biomechanics of walking in early humans by using comparative anatomy, including that of bears.
Based on footprints collected from semiwild chimpanzees at Ngamba Island Chimpanzee Sanctuary in Uganda and two captive juveniles at New York’s Stony Brook University, the scientists found that chimpanzees have relatively narrow heels compared to their forefeet, a trait shared with bears. But the Laetoli footprints, including those at Site A, came from an animal with wide heels relative to their forefeet.
The Site A footprints also contained the impressions of a large hallux (big toe) and smaller second digit. The size difference between the two digits was similar to that of humans and chimpanzees, but not black bears. These details further demonstrate that the footprints were likely made by a hominin moving on two legs. But in comparing the Laetoli footprints at Site A and the inferred foot proportions, morphology and likely gait, the results reveal that the Site A footprints are distinct from those of Australopithecus afarensis at Sites G and S. That means, according to the report published in the journal Nature in December 2021, that there were different hominin species walking bipedally on this landscape—but in different ways and on different feet.
Our initial walks in trees
It’s hard to tell when—and why—our ancestors climbed down from trees and started walking on two legs. Many early hominins capable of bipedal walking were also well-adapted for climbing, and we lack fossil evidence from a key period when climate change turned forests into dry, open woodlands called savanna mosaics, which might have pushed hominins onto the ground. Now a study on modern chimpanzees could help fill in the gaps. Scientists observing chimpanzees in the Issa Valley, Tanzania, have shown that despite living in a savanna mosaic, the apes frequently climb trees for valuable food, potentially explaining why early hominins kept their arboreal adaptations.
Despite living on an open terrain, chimpanzees in Tanzania still frequently climb trees to gather fruit and other foods found high in the canopy. This suggests that bipedalism may not have evolved purely as a response to ground-based travel, but also for efficient and safe movement within trees.
Issa Valley is divided between a small number of thick forests surrounding riverbanks and open woodlands. The chimpanzees forage more in the woodlands during the dry season, when it offers more food. Their diet and habitat are comparable to those of some early hominins, which means their behavior might offer insights into those extinct animals’ lives.
Previous research had shown that, compared to chimpanzees living in forests, Issa Valley chimpanzees spend just as much time moving in the trees. Researchers from the Max Planck Institute for Evolutionary Anthropology wanted to test if something about how they foraged could explain their unexpectedly high arboreality. Savanna mosaics are characterized by more sparsely distributed trees, so they hypothesized that adapting behavior to forage efficiently in a tree would be especially beneficial when the next tree is farther away.
For their new study, the scientists monitored the adults of the Issa community during the dry season, watching how they looked for food in the trees and what they ate while there. The heights, shapes and sizes of the trees were recorded, as well as the number and sizes of branches.
Chimpanzees move within trees not by climbing on thin branches but by hanging under them or standing upright and holding onto ones nearby. Although these safe behaviors are usually associated with foraging in dense forests, they’re also important when searching for food in a savanna mosaic.
Publishing their findings in the journal Frontiers in Ecology and Evolution in July 2025, the scientists say they discovered that the Issa chimpanzees mostly ate fruit, followed by leaves and flowers—foods found at the ends of branches, so the chimpanzees needed to be capable climbers to reach them safely. They foraged longer in trees that were larger and offered more food. The longest foraging sessions—and the most specialized behaviors for navigating thinner, terminal branches—were seen in trees with large, open crowns offering lots of food: perhaps abundant food justified the extra effort and time. A similar trade-off between the nutritional benefits of specific foods and the effort of acquiring them could also explain why chimpanzees spent more time in trees while eating nutritionally-rich, hard-to-access seeds.
Because they are relatively large, chimpanzees move within trees not by climbing on thin branches but by hanging under them, or standing upright and holding onto nearby branches with their hands. Although these “safe” behaviors are traditionally associated with foraging in a dense forest, these findings show they’re also important for chimpanzees seeking food in a savanna mosaic.
In conclusion, the researchers suggest our bipedal gait continued to evolve in the trees even after the shift to an open habitat. Observational studies of great apes demonstrate they can walk on the ground for a few steps, but most often use bipedalism in the trees. It’s logical that our early hominin relatives also engaged in this kind of behavior, where they could hold onto branches for extra balance. If Issa Valley chimpanzees can be considered suitable models, bipedal and suspensory behaviors were likely vital for fruit-eating, large-bodied, semiterrestrial hominins to survive in an open habitat.
Plant life changed toward the end of the Cretaceous Period, with flowering plants, known as “angiosperms,” creating more diverse habitats on the ground. Many mammals adopted more ground-based lifestyles, even before an asteroid killed the dinosaurs.
Our calling to the ground didn’t depend on dinosaur die-off
So, although we can’t say exactly when and why our ancestors descended from the trees and started walking on two legs, we do know that more mammals were living on the ground several million years before the mass extinction event that wiped out the dinosaurs. New research, led by England’s University of Bristol and which was published in the journal Palaeontology in April 2025, provides fresh evidence that many mammals were already shifting toward more ground-based lifestyles leading up to the asteroid’s impact.
For some time, it’s been known that plant life changed toward the end of the Cretaceous Period, with flowering plants—known as angiosperms—creating more diverse habitats on the ground. We also know that tree-dwelling mammals struggled after the asteroid impact. What had not been documented, however, was whether mammals were becoming more terrestrial, in line with the habitat changes.
While the authors of previous studies had used complete skeletons to study ancient mammal movement, the University of Bristol researchers are some of the first to use small bone elements (specifically, ends of limb bones) to track changes within an entire community rather than just individual species. Ends of limb bones were analyzed because they bear signatures of locomotory habits that can be statistically compared with those of modern mammals. By examining the small, fossilized bone fragments from marsupial and placental mammals found in Western North America from museum collections in Calgary, California and New York, the science team discovered signs that these mammals were adapting to a life on the ground.
I’d say that the human “urge for going” is rooted in trees.
These results mean that the vegetational habitat was more important for the course of Cretaceous mammalian evolution than any influence from dinosaurs, and they offer new insights into how prehistoric mammals responded to changing environments—a few million years before the asteroid impact reshaped life on Earth.
Our travels, from the top down
Early on, it seems, our ancestors developed the urge for going—on two feet and from the top down. Science bears that out.
But the thought that it may all have started in the trees—some of my favorite beings—makes the human history of traveling all the more enchanting.
Here’s to finding your true places and natural habitats,
Candy
