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A Closer Look At: Orrorin [1]
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Date: 2025-05-27
Orrorin is another contender for the title of “oldest known hominin”, but like Sahelanthropus it is steeped in controversy.
This is a diary series in which we take a closer look at hominins, fossil ancestors of the human family.
Orrorin fossils photo from WikiCommons
The Lukeino Formation, a bed of sediments in the Tugen Hills in Kenya, has long been known as a good place to find fossils of ancient cercopithecid Old World monkeys, and a number of teams have carried out excavations there over the decades. But in 1974, Martin Pickford found a single molar tooth in these deposits which had every appearance of being from a hominin.
Years later, in August 2000, Pickford was leading another team in the Tugen Hills with French researcher Brigitte Senut in an area known as Kapsomin. Kiptalam Cheboie, one of the Kenyan field workers, was walking to the base camp through a previously-unexplored dry stream bed when he spotted a fragment of jawbone with some teeth eroding out of the ground. They looked humanlike. A few days later at the same spot the team found four pieces from two thighbones, a fragment of upper arm bone, and some teeth and small finger bones. In all, 13 pieces of bone were found, from at least 2 individuals and perhaps as many as 5. None of the skull was found. The 1974 molar tooth may or may not also be from Orrorin.
The fossils were in a layer just below some volcanic deposits which the team had already dated to 5.65 million years, making them close to 6 million years old. If they were really hominin, this would make them the oldest known human ancestors at the time, pushing back the story of human evolution by a million and a half years.
Because the fossils had been discovered in the year 2000, Pickford nicknamed the find “Millennium Man”. Since the bones had been found in a dry stream bed, Pickford’s first concern was that they had perhaps been washed out from younger sediments and re-deposited in the older layers. But the layer in which the hominin had been found also contained the remains of pigs, horses, and monkeys which all corresponded to known species that had already been dated to around 6 million years. Some of these bones bore carnivore tooth marks, indicating that this had probably been a den into which ancient big cats, likely leopards, were dragging their prey—including the hominins.
Pickford gave the new find the name Orrorin tugenensis. The word orrorin came from the local Tugen people, and referred to the “original man” around which their creation story was based.
The first priority was the dating. Radiodating using the potassium-argon method returned an age for the fossil layer of 5.8 to 6.2 million years, confirming the age indicated by the associated mammal fossils. This was significantly older than Ardipithecus ramidus, then the oldest-known hominin, which dated to 4.4 million years.
Although no skull bones were found, there were several pieces of jaw and teeth. The jaws were roughly the same size as other early hominins like Sahelanthropus, implying that Orrorin had approximately the same brain volume—around 375 cubic centimeters. The molars were small and squarish with a thick enamel, similar to humans, while the front teeth seemed to be more apelike. The canine was reduced in size, however, and the wear pattern seemed to show no signs of the honing arrangement found in apes, marking it as a probable hominin. The shapes of all the teeth implied a varied diet that was heavy on plant material, mostly fruits and berries but also seeds and perhaps insects.
The finger bones were slender and curved, suggesting that they were adapted for climbing and that Orrorin spent a lot of time up in the tree branches. This was confirmed by the ulna bone, which had a large surface for the attachment of powerful muscles—good for gripping branches.
Most significant of the Orrorin finds, however, were the pieces of two femurs from the area near the hip joint—one of which had an intact ball. In the absence of a skull, knee joint, or foot bones, this was the most important source of information about how Orrorin moved around.
In apes, which move quadrupedally with the spine held horizontally, the pelvis is long and narrow. The femur is joined to the hip by a ball-and-socket joint, and in apes this is very mobile with a relatively small “ball”, allowing for the use of the grasping feet while climbing. In humans, by contrast, the pelvis has to hold up all the weight of an upright body, so it is short and wide, forming a bowl shape that supports the trunk, and the femur joint is strengthened and stiffened.
In Orrorin , the ball of the femur was larger than that of both apes and australopithecines, the “neck” of the femur was longer, and the upper portion of the femoral shaft was wider. X-ray CT scans indicated that the interior of the bone had been thickened in places, something that happens as the living bone grows and responds to the stresses placed on it. In particular, in bipeds there tends to be a lot of stress on the neck of the femur, causing the upper part to become thinner and the lower part to become thicker. And this is the pattern we see in Orrorin. Critics, however, have pointed out that this trait can also sometimes be seen in non-bipedal primates.
The neck of the femur also seems to be set to the proper configuration to allow it to angle inwards, which allows the knees and feet to be placed directly under the center of gravity—an adaptation seen only in bipeds.
After intense study, Pickford’s team concluded that the femur fossils indicated that Orrorin was a biped, with bone structure that was more similar to humans than apes.
Since 2001, a number of additional studies have been done on the bones, and they all reached the same conclusion: Orrorin was at least partially a biped, and while it was on the ground it was capable of walking on two legs. It may, however, have had its own unique way of walking which, though effective and functional, was biomechanically a bit different than later hominins. This was probably to be expected, since Orrorin still had many of the anatomical traits of apes, and “bipedalism” was at the time a new evolutionary experiment.
But then Pickford’s team went even further, and concluded that some of the subtle anatomical traits on the teeth and hip joints seemed to be even closer to those found in the later Homo genus than to the earlier Ardipithecus and Australopithecus. In effect, Pickford was putting out the view that Orrorin had been a more efficient biped than these other genera, that the Homo bipeds had perhaps evolved directly from the root represented by Orrorin and that none of the ardipithecines or australopithecines may have been in the direct human lineage but were just side branches. In a stroke, Pickford had removed the relevance of nearly all of the early hominin fossil record. Needless to say, this was not a hypothesis that was received gracefully: it provoked a storm of controversy and, not surprisingly, has not been widely accepted. Many have pointed out that evolution does not run forward in a straight line: human ancestry is very “bushy” and has a lot of contemporaneous side shoots that die out without descendants, and many hominin species have demonstrated a mosaic mixture of some humanlike traits and some apelike. And of course lots of entire careers have been based on the hypothesis that Ardipithecus and Australopithecus are ancestral to early Homo and Homo sapiens, and nobody wants to see that apple cart upset. Since there are no current indications of any other fossil links between Orrorin and Homo, removing those genera from the line of ancestry would essentially open a huge gap in the hominid timeline and take us all the way back to square one. And of course the flip side to this entire argument is that some researchers have concluded that Orrorin was not a hominin at all, or may be a hominin but is itself not in our direct line.
Orrorin did change the accepted narrative of human evolution in another way, though. About ten million years ago, the climate in east Africa began to dry out and become cooler. The tropical forest that had covered most of the continent began to shrink, and large patches of savannah grassland began to appear. It had long been assumed that this was a trigger for the appearance of bipedal humans. The forest apes, the reasoning went, had been forced from the trees and pushed out onto the treeless savannah, and bipedalism was an evolutionary response to this new habitat, allowing hominins to move more efficiently. And this had happened very quickly.
Study of the ecological niche in which Orrorin lived, however, indicated that this habitat was still largely covered by forest, and that Orrorin itself still had a number of adaptations for living in trees. Bipedalism, it seemed, predated the transition from tree-dweller to savannah-dweller. One possibility that has been proposed is that the ape ancestors of hominins were already “pre-adapted” to an upright posture, by habitually moving through the trees by brachiating (swinging vertically under the branches) or by walking erect along the top of large branches—something modern Orangutans, Gibbons and Siamangs still do.
There was, finally, some drama with the fossils which, given the air that surrounds them, seems only fitting. While being studied, the Orrorin fossils were kept at the National Museum of Natural History in Paris. In the meantime, a museum was built in the village of Kipsaraman in Kenya, near the place where they were found, and the fossils went on public exhibit. But after a short while the museum closed down, and the Orrorin fossils are now, according to government officials, stored away in a secure bank vault somewhere in the capitol city of Nairobi.
In 2022, Pickford again stepped on some paleontological toes when he named a new species of Orrorin. This centered on a partial jaw that had been found in 1985 in the Magabet Formation in Kenya, dating to around 5 million years. Originally named Homo antiquus praegens (“ancient man who came before”), it had been later reclassified as a specimen of Ardipithecus ramidus. But when Pickford found a number of new fossils in the Magabet in 2005, he re-studied the earlier jaw and concluded that it was very similar to Orrorin, though it differed in some details. In 2022 he reassigned it, along with the handful of new fossils that he had found, as Orrorin praegens, and considered it to be a later species of Orrorin. A number of taxonomists, however, have argued that the specimens are not new and should remain within Ardipithecus.
There does seem to be a consensus today that Orrorin tugenensis was a bipedal hominin and that it was probably ancestral in some way to the ardipithecines. A minority of authorities, however, remain unconvinced by the fragmentary fossils.
More fossils would help solve the issue. But since the “millennium man” was found in 2000, there have been no further discoveries, so all we have for now are the handful of original bones, locked away in a bank vault in Nairobi.
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