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A Closer Look At: Australopithecus anamensis [1]

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Date: 2025-10-28  

Australopithecus anamensis has been at the center of a number of scientific debates.

This is a diary series in which we take a closer look at hominins, fossil ancestors of the human family.

Australopithecus anamensis skull photo from WikiCommons

In 1965, a team of paleontologists led by Bryan Patterson, from Harvard, was excavating at a site on the western shore of Lake Turkana in northern Kenya known as Kanapoi when they uncovered a small fragment consisting of the end of a humerus upper arm bone. Catalogued as KNM-KP 271, the bone had every appearance of being from a hominin (and the site had already yielded finds from several different australopithecines). But because it was so small and the dating was uncertain (his first estimate was 2.5 million years, but this was quickly revised to around 4 million years, based on the fossil animals with which it was found) there were not enough details to enable an identification of which species it represented.

It wasn't until 1994, when the political situation inside Ethiopia stabilized after years of civil war, that another team led by Meave Leaky returned to the Kanapoi site. Meave was part of a family dynasty which had dominated the paleo-anthropology world for decades. Louis and Mary Leaky had been the first to excavate in East Africa back in the 1960s and had made several important finds. Meave married their son Richard, and together they also made a number of significant discoveries. The Leaky team, known informally as “The Hominid Gang”, played a crucial role in training a number of native Africans to carry out paleontological work in their home countries, and was also where many new American and European researchers gained their first field experience.

Both Louis Leaky and his son Richard were strong personalities with strong opinions, and they were often mired in debates and conflicts, many of which became personal. In later years Richard became the director of the Kenya museum system, which gave him authority to decide which teams would be allowed to excavate at what sites—and this often led to additional rancor in what was already a highly contentious field. The Leakys made many important discoveries, but they also made many enemies in the personal politics of the paleo-anthropological world.

Just two weeks after arriving at Kanapoi, Meave Leaky's team made their first discovery: an upper palate with teeth that was catalogued as KNM-KP 29283. The sediments here were sandwiched between two layers of volcanic ash which could both be reliably dated, fixing their age between 3.9 and 4.2 million years ago. This was a time period from which only fragmentary and scrappy hominin fossils had previously been found, so Meave knew that this bone was significant, and was likely the oldest hominin to be found so far (she was unaware that Tim White's team was at that same time already excavating Ardipithecus).

A number of further finds followed, consisting of several teeth, some upper and lower jawbones and some leg bones. Some of the significant fossils were catalogued as KNM-KP 29281, a lower jaw with teeth, and KNM-KP 29285, pieces of a tibia lower leg bone.

By the end of 1994, though, Meave had received word that Tim White was working on Ardi, which was slightly older, and she met up with him to compare her new fossils with those found by White to see if they were the same species. After examining both finds, they concluded that the bones from Kanapoi were different from White's Ardi, and that they belonged, along with Patterson's earlier ulna fragment, to a new species which Meave named Australopithecus anamensis (from the local Afar word “anam” and meaning “southern ape from the lake”).

Since that time new anamensis finds have been made, with a number of additional bones, including teeth and a femur, coming from nearby Allia Bay. In 2016 Ali Bereino, a native herder at Waranso-Mille, found a fragment of bone while digging a hole in his goat pen and gave it to researcher Yohannes Haile-Selassie, who promptly investigated the site and was able to recover a nearly-intact skull dated at 3.8 million years. Known as the “MRD skull”, it belonged to an adult male with well-worn teeth.

Today we have around 100 fossil bones from at least 20 anamensis individuals from several sites around Lake Turkana, enabling us to piece together much of the skeleton. The fossils indicate that Australopithecus anamensis was a fully bipedal hominin standing about five feet tall, and which appeared to be intermediate in both anatomy and timescale between the famous “Lucy”, Australopithecus afarensis, and Ardipithecus ramidus.

From the type of sediments in the area and from the collection of animals that had been fossilized alongside anamensis, Meave was able to conclude that Kanapoi had once been the floodplain of a river that meandered for several miles before draining into the ancient Lonyunum Lake, of which modern Lake Turkana is just a remnant. The fossils, which included things like giant warthogs, sabertooth cats, monkeys, hyenas, and elephant ancestors, seem to have come from a variety of different habitats, however, ranging from closed-canopy forest to patches of open grassland. This indicates that anamensis was a generalist and was becoming adapted to life in different habitats.

The reconstructed skeleton of A anamensis clearly indicates that it had many traits that were more humanlike than apelike. The teeth have thicker enamel and the canines are reduced in size compared to the earlier hominins, though the incisors are broader and flatter as in apes. The brain volume is measured to be around 375cc, which is larger than previous hominins, and the muzzle is less pronounced while the cheekbones project forward, which are humanlike characteristics. The front of the lower jawbone, however, is slanted sharply backwards like an ape's, rather than being vertical like a modern human.

The location of the foramen magnum demonstrates a bipedal stance, and this is confirmed by several other bones. The tibia bone in the shin is flared at the upper end into two humanlike hollowed knobs known as “condyles”, an adaptation for upright walking which allows a single leg to support the entire body weight when striding. The lower end of the tibia is also humanlike in shape where it meets the anklebones, and is thickened to absorb the shock of bipedal walking. And several vertebrae indicate that the spine was held vertically as in a biped, rather than horizontally as in a quadruped.

Nevertheless, there are also indications that anamensis still spent a lot of its time in trees. The long humerus and ulna bones indicate that the shoulder and elbow joints were loose and mobile as in apes, which allowed for flexibility while climbing. At the same time, the bones show attachment points for large and strong muscles in the forearms and upper limbs, and also long curved fingers with powerful tendons, all of which would have given a powerful grip useful for climbing.

Anamensis therefore seems to have been equally at home in the trees or on the ground, which was apparently an adaptation to its mixed forest/grassland habitat. The jaws and teeth were still apelike in many ways, with parallel sides to the lower jaw and tooth shape, and microscopic examination of the scratch-marks on the enamel as well as isotopic analysis indicates a diet of herbaceous plants and leaves with a mix of some grasses and possibly some nuts or tubers rather than primarily soft ripe fruits, indicating that these hominins probably obtained much of their food from the ground, like a gorilla.

The bones may also give clues to their social structure. Unlike later hominins, the male anamensis seem to have been larger than the females and had proportionately larger canines. This is typical of social organizations like those of gorillas or chimps, where the males have a hierarchical dominance structure which determines who has access to the females. However, the canines are much smaller than in earlier hominins, which is a human trait and which indicates more of an egalitarian social structure with steady pair-bonds. Anamensis may have had a social structure that lay somewhere in between.

In its body structure, then, Australopithecus anamensis exhibits a mosaic mixture of both modern and archaic traits, and appears to be intermediate between the earlier ardipithecines and later Homo.

One crucial unanswered question, however, centers around the structure of its foot. As we have seen in a previous diary, the Ardipithecus ramidus skeleton, dating from 4.4 million years ago, indicates that it was bipedal and could move along the ground on two legs, but it still retained the movable grasping big toe found in modern apes. The Australopithecus prometheus “Little Foot” bones show a similar structure. This trait, however, had been lost by the time of Australopithecus afarensis, since Lucy's species, from 3.7 million years ago, shows an in-line big toelike that found in modern humans. So, logically, it looks like hominin locomotion had made a major change between these two species, from a grasping big toe that was good for climbing to a straight big toe that was exclusively adapted for obligate bipedal walking—and since A anamensis, at 4 million years, fell inside this time gap, it could be assumed that the foot anatomy of anamensis could give us a great deal of information about how this change occurred. Unfortunately, however, none of the relevant foot bones have been found for anamensis, and we simply do not know anything about how this hominin's foot was structured, or what type of big toe it had.

In addition, the matter was confused further in 2012, when a partial foot skeleton was found at Woranso-Mille in Ethiopia. This find dated to around 3.4 million years—at the time of Lucy's species—but it differed significantly from Australopithecus afarensis and was given the name Australopithecus deyiremeda. Most significantly, deyiremeda clearly had a divergent big toe in its foot, similar to Ardipithecus. If anamensis also had a grasping toe, it could therefore mean that there must have been another currently-unknown hominin between A anamensis and A afarensis, and that the transition from grasping foot to walking foot happened even more rapidly than we currently believe. Or, it may even indicate that neither Ardipithecus nor anamensis is actually in the human line and are just side branches, and that obligate bipedality may have evolved from some other currently-unknown hominin.

While, as we will see in a later diary, the taxonomic status of deyiremeda is disputed, the find also indicates that there were more than one hominin species living in East Africa at this time. This should perhaps not have been a surprise, since most animals evolve through branches and multiple radiations, leading to a family tree that is bushy and branchy, with many dead ends, and not the neat and tidy single line of descent that is often pictured in popular science books.

The ecological zone in which the fossils have been found also supports the idea of a multiplicity of species. Geological and fossil evidence indicates that anamensis lived in a patchwork environment with areas of closed-canopy forest intermixed with areas of savannah and scrubland, along with scattered lakes and rivers. These fragmented habitats are perfect for producing and supporting scattered populations which become more and more specialized in their particular local niche and thereby are evolutionarily isolated and evolve into new species, a process known as “adaptive radiation”.

The existence of multiple hominin species at the same time, then, raises the possibility that we will never be able to determine exactly which species is related to which, or which ones are in our direct line and which are just closely-related cousins who are on a side branch. And indeed the exact position of Australopithecus anamensis on the human family tree is still a matter of intense debate.

Almost from the time of its discovery, anamensis was assumed to be an evolutionary descendant of Ardipithecus ramidus, and to itself be the evolutionary ancestor of Australopithecus afarensis. But there have always been dissenting hypotheses. A few experts pointed to the similarity between the projecting cheekbones and sagittal keel (a thickened ridge at the top of the skull where the jaw muscles are attached) of anamensis and the Paranthropus genus, and concluded that the MRD skull is ancestral to that side branch of hominins, or perhaps split into several species which included members of both branches. Some other authorities conversely argued that anamensis and afarensis are actually the same species, and these are sexual differences. But those views were very much a minority opinion, and the consensus still held that anamensis evolved over time into afarensis.

There were also questions, however, provoked by the date assigned to the MRD skull. Although it was identified as an Australopithecus anamensis, the skull was dated to 3.8 million years ago—a time when Australopithecus afarensis is also known to have been living in East Africa. That led to a debate, as some critics pointed out that the fact that both lived together at the same time for several hundred thousand years indicates that they are the same species, while others asserted that they were different species but that anamensis could not be the ancestor of afarensis.

The debate was muddied even further when it was pointed out that even if the two species co-existed for a long time, that did not necessarily mean they were not ancestors-descendants. Many species in a changing environment do indeed adapt slowly over time throughout their range into a different species, in a linear process that is known as “sympatric speciation”. However, a large proportion of speciations also happen when a small portion of a population becomes cut off and isolated from the rest, usually by a geological barrier like a river or mountain range or by an ecological difference like different mating habits or diet. Such a process is particularly common in populations which have become fragmented by different ecological niches—as we know anamensis was. In these circumstances, the isolated population can differentiate into a new species that can no longer interbreed with the old one, even though both populations continue to exist—a branching process that is known as “allopatric speciation”. It was therefore, some experts pointed out, entirely possible that an isolated population of anamensis evolved into afarensis while the rest of the main anamensis population continued unchanged until it went extinct.

Then in 2022 a new wrinkle appeared, when a team carried out the new process of cosmogenic nuclide dating on some Australopithecus africanus fossils from South Africa and found that they were over a million years older then previously thought—and suggested the possibility that africanus, not afarensis, was the evolutionary descendant of anamensis, making Lucy and her entire species another side branch. That hypothesis has not been widely accepted yet either.

Finally, in 2024 the murky waters became even more unclear, when an anamensis mandible that had been found in Kenya in 2011 was dated to 4.3 million years—overlapping with the time that Ardipithecus ramidus was alive, and re-creating the same ancestor-descendant problem at the other end of the time span. So now the argument rages as to whether the ancestor of anamensis was Ardipithecus or Sahelanthropus or Orrorin, or something else entirely that we currently know nothing about. And of course the probability is good that future fossil finds will expand the time ranges for the known species even further in both directions.

The debates will therefore likely continue until we find more fossils which answer these questions, and we can in all likelihood assume that much of what we currently “know” about Australopithecus anamensis and its relation to other species will turn out to be mistaken or incomplete.

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