The two figures come from different joints. That difference is the entire argument. In a climbing chimpanzee, the upward flexion of roughly 45 degrees happens at the ankle [4]. In a sooty mangabey, whose ankle lacks that mobility, it happens further forward: Luke Fannin measured as much as 46 degrees in the midfoot as the animals went up narrow trunks [6][7].
The one-degree difference between those two numbers [16] should not be read as the monkey winning. Both are maxima, both come from animals moving in different bodies, and the honest reading is the one Fannin gives, which is functional equivalence rather than superiority [9]. Set against a human ankle's roughly 20 degrees [5], the mangabey midfoot bends about 2.3 times as far [15].
What this does to paleoanthropology is narrower than a reconstruction. It is also more useful. The inference under pressure has the logical form of a necessary condition: some accounts hold that an ape-like foot was required for effective vertical climbing, while others allow a more monkey-like ancestor [12]. A single living animal doing the thing without the trait is enough to retire the requirement. It is not evidence about what the last common ancestor did, only the removal of one reason to say what it could not have done, and that distinction matters most precisely when a fossil foot is the only foot available.
This measurement leaves open how often, or at what cost. A peak joint angle during a successful climb measures none of force, tendon stress, energetic expense, or margin for error; reaching 46 degrees of midfoot flexion shows only that the movement is possible, not that it is as economical or as safe as a chimpanzee's climb. The Ohio State release also does not give the number of animals or climbing bouts behind the measurement [17], so the shape of the distribution behind that maximum is not yet public. Those are questions for the paper in the Proceedings of the National Academy of Sciences [3], not objections to it.
The design deserves credit on its own terms. These were wild animals climbing for their own reasons, foraging, hiding, or heading for a sleeping site [13], filmed at the Taï Forest Monkey Project field station in Ivory Coast [2]. Senior author W. Scott McGraw's point is that the video resolves what the joint is actually doing under load, where before the field had hunches [10]. Monkeys had long been cast as arm-driven climbers [11], and that assumption was doing quiet work in arguments about a divergence 6 to 7 million years old [8] and about how habitual bipedalism, which only humans practice regularly, got started [14]. My view: the necessary-condition claim is spent. The positive case for a vertical-climbing ancestor stands exactly where it stood yesterday.
