Ardipithecus ramidus is one of the most complete hominid skeletons found so far, and the earliest one to be unequivocally accepted as a bipedal human ancestor.
This is a diary series in which we take a closer look at hominins, fossil ancestors of the human family.
In 1981, a group of paleontologists who had been working at the fossil site in the Middle Awash area of Ethiopia made a side trip to prospect another potential fossil site known as Aramis. They found a few fragmentary bones.
It wasn’t until 1992 that another team returned to Aramis. In particular, they wanted to look at some deposits which lay between two volcanic ash layers, dated to around 4.4 million years. They found fossils of monkeys and kudu antelope.
In December, though, a group led by Japanese paleoanthropologist Gen Suwa and American Tim White uncovered some teeth and a jawbone, and a year later found some more teeth and an arm bone in the same sediment layer. They were described and published as a new species of hominin, Australopithecus ramidus (the name ramidus means “root” in the local Afar language).
In 1994 the team returned to Aramis, and on their first day of digging, 175 feet away from the original find, Ethiopian researcher Yohannes Haile-Selassie uncovered a hominin hand bone, which quickly turned into a collection of over 100 bones from a single individual in the same 4.4 million year old layer. This assembled skeleton, which was given the catalogue number ARA-VP-6/500, demonstrated some significant differences from the known Australopithecus genus, so it was re-assigned to a new genus Ardipithecus (the name means “ground ape”). A year later, paleoanthropologist Sileshi Semaw found some more bones, from around nine individuals, at the nearby site of Gona, which dated between 4.32 and 4.51 million years, and they were also assigned to this new species. So far, fossils have been recovered of around 35 different individuals, both male and female.
“Ardi”, as the skeleton became known, was an adult female measuring about four feet tall and weighing probably around 100-110 pounds. About 50% of the skeleton was found, making her the best-represented of the early hominins (more complete than the famous “Lucy”) and allowing us to reconstruct much of what she looked like.
It came as a surprise.
There are only a tiny handful of existing fossils of ancient ape ancestors that have been found. In general, animals from forested environments, like apes, do not tend to fossilize very often because the wet acid soil destroys bones before they can be preserved. Only a few teeth of probable ancient Chimpanzee ancestors have been found, and nothing at all is definitely known of ancient Gorillas. In the absence of good evidence, then, many authorities simply assumed that the last common ancestor between humans and apes must have looked very much like a modern knuckle-walking Chimpanzee or Gorilla (our closest living relatives) and must have had a similar lifestyle. But Ardi showed that this was not true, and that the last common ancestor had a distinct lifestyle of its own that was neither Chimp nor Human, and the reconstructed skeleton showed that Ardi had a peculiar mix of traits that were both apelike and humanlike. This should perhaps have not really been a surprise, since modern apes are very specialized for their forest ways of life and their knuckle-walking method of locomotion, and they have evolved just as long and just as far away from our common ancestor as we have.
The Ardipithecus skull in particular, though badly crushed, demonstrated both apelike and humanlike traits when it was reconstructed. The brain is chimp-sized, measuring around 350cc in volume, and the face projects forward like an ape, though not as sharply as in modern apes. The brow ridges, though smaller than those in Sahelanthropus, are still heavy and prominent, as in apes, and there is sharp nuchal ridge at the back of the skull where the neck muscles attach, as in apes. The molar teeth are still relatively small, though slightly larger than in apes, and have relatively thin enamel, indicating a diet of mostly soft foods, though Ardi did not have the large specialized incisor teeth that are typical of fruit-eating primates. It was probably an omnivorous generalist, eating everything from fruit to nuts to insects.
There are, however, humanlike characteristics as well. The canine teeth in both males and females were smaller than those found in apes, and this is typical of hominins. There is also no evidence of the “honing” mechanism found in apes, in which the canine teeth are constantly sharpened by rubbing against the lower premolars.
The most important feature, however, is the foramen magnum—the hole through which the spinal cord enters the skull. In apes, with their quadrupedal gait, the spine is held horizontally and the spinal cord enters at the very back of the skull. In bipedal humans, though, with their upright stance and vertical spine, the spinal cord enters at the very bottom, allowing the skull to balance atop the neck. And Ardi’s skull clearly shows a foramen magnum that is shifted towards the bottom, as with a biped.
This was supported by a number of humanlike traits seen in the rest of the mostly-apelike skeleton. In quadrupedal apes, the bones of the lower legs are in a straight line from the hip through the knee joint to the feet. In bipedal humans, however, the feet have been shifted inwards to place the center of gravity between the hip joints and to allow proper balance for two-legged walking. Since the knee joints are therefore closer together than the hip joints, the thigh bones (the femur) must slant inwards from the hips to the knees, and this forms a distinct angle between the femur bone and the knee joint which is only found in bipeds. There are some indications that this angled joint can be found on Ardipithecus, though the incompleteness of the fossils and their heavily damaged state make this uncertain, and some experts have questioned it.
The pelvis also shows some signs of an upright animal (in addition to indicating that Ardi had the relatively flaring pelvis of a female rather than the narrow hips of a male). In quadrupedal apes, the pelvic girdle is long and narrow. In bipedal hominins, however, (and in Ardi) the pelvis is short and wide, and is oriented upwards to help hold the weight of the upright torso. The ball of the hip joint in Ardi’s femur bone is also closer to that of humans than to apes. There were also indications of attachment points (known as the anterior inferior iliac spine) for strong leg muscles that were used for bipedal walking.
Taken together, these adaptations all point to an animal that may have spent at least some of its time on the ground, and was probably able to move about on two legs while doing so.
These uniquely hominin traits, however, were accompanied by a number of anatomical features which also indicate an apelike structure and a lifestyle that was still largely arboreal. The skeleton shows that Ardi had long arms and mobile shoulder joints, which are useful for climbing in trees. Oddly, the proportion of the arms did not match either those of modern apes or humans, indicating that this trait evolved further within both sides some time after the split. In addition to the arms, the fingers were long and permanently curved—an adaptation for grasping tree branches. There was no sign, however, of the stiffened wrist or finger bones which are found in modern knuckle-walking apes.
Most important, however, were the feet. The long bones of the foot had an arch as in humans (which apes lack), though this was not as well-developed in Ardi as in later hominins. On the other hand, Ardi’s skeleton clearly shows that the big toes on the feet (known technically as the hallux) were large and mobile, and stuck out from the side of the foot like a human thumb—a feature still seen today in modern apes. This divergent big toe was important for climbing around in tree branches, and indicates that despite being capable of bipedal walking while on the ground, Ardi was still mostly a creature of the trees, perhaps only descending to the ground occasionally to hunt for food. She must have had her own unique method of bipedal walking that was different from all the known later hominins, and since subsequent australopithecines (like Lucy) lacked this primitive divergent toe, it must have been lost quickly in their evolutionary development, as soon as the hominin line had committed itself to spending a lot of time moving around on the ground.
The team also recovered around 6,000 bones and bone fragments of other animals at the site, which allowed them to make a fairly detailed reconstruction of the local environment within which Ardi lived. Among the species which lived alongside Ardi were monkeys and kudu, early elephants and hyaenas, porcupines, a variety of rodents and bats, and fossilized wood and seeds. Taken together, they indicated that Ardipithecus lived in an area that was mostly forested but also contained some open areas.
The fact that only a handful of these hominin fossils were found also indicated that Ardipithecus was probably a quite rare animal and did not have a large population. It probably lived in small scattered troops. The males and females were about the same size and weight, indicating that their social structure may have revolved around bonded pairs and would have been more egalitarian like modern Bonobos rather than more male-dominated like Chimpanzees.
For decades, paleoanthropologists had hypothesized that the switch from an arboreal to a ground-dwelling bipedal lifestyle had been the result of ecological changes brought about by a drying and cooling climate, causing forests to shrink and driving the earliest hominins out onto the open savanna grasslands. But Ardi showed that this was probably not the case, and that the earliest hominins, although not yet fully bipedal, had nevertheless already begun developing bipedal locomotion before this ecological shift had taken place. It also appears from the Ardi skeleton that humanlike teeth, such as smaller canines and thicker enamel, had already begun to appear in hominins before bipedal walking had been fully developed.
A few authorities have concluded that Ardi is an ape ancestor rather than a hominin, but as of 2025 the majority consensus seems to be that A kadabba is likely the precursor to A ramidus, and that the genus Ardipithecus seems to be somehow ancestral to the australopithecines. Some have considered A ramidus to be directly ancestral to Australopithecus anamensis, the earliest known australopithecine, but an anamensis jawbone found in 2011 has been dated to 4.3 million years, which overlaps in time with the youngest ramidus finds. This has led others to conclude that ramidus is a side branch and not in the direct line to humans at all, while still others have postulated that the ramidus species may have become split into two distinct populations by some geographic or ecological barrier, with one of these populations then going on to evolve into Australopithecus, a process known as “allopatric speciation”.
In addition, some researchers have considered Ardi to be so similar in its skeleton to Orrorin and Sahelanthropus that all three should be grouped into the same genus, which under the rules of nomenclature would be Ardipithecus. This view has not been widely accepted.
NOTE: As some of you already know, all of my diaries here are draft chapters for a number of books I am working on. So I welcome any corrections you may have, whether it’s typos or places that are unclear or factual errors. I think of y’all as my pre-publication editors and proofreaders. 😉
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