They also found that knuckle walking evolved at least two different times; gorillas fundamentally knuckle walk differently than chimpanzees and bonobos.
Read more on Science Daily: Bipedal Humans Came Down From The Trees, Not Up From The Ground
Kivell and Schmitt think this suggests independent evolution of knuckle-walking behavior in the two African ape lineages.
Some scientists point to features in the human anatomy as our own vestiges of a knuckle-walking ancestry. One notable example is the fusion a two wrist bones that could provide us extra stability, a feature we share with gorillas, chimps and bonobos.
But some lemurs have that feature too, and they do a variety of different movements in the trees but do not knuckle-walk, Kivell said.
Altogether, the evidence leans against the idea that our own bipedalism evolved from a knuckle-walking ancestor, the pair wrote. "Instead, our data support the opposite notion, that features of the hand and wrist found in the human fossil record that have traditionally been treated as indicators of knuckle-walking behavior in general are in fact evidence of arboreality."
In other words, a long-ago ancestor species that spent its time in the trees moved to the ground and began walking upright.
There are no fossils from the time of this transition, which likely occurred about seven million years ago, Kivell and Schmitt said. But none of the later fossils considered to be on the direct human line were knuckle-walkers.