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Australopithecus anamensis
Australopithecus anamensis

[Image: 640px-Australopithecus_anamensis_5476.JPG]

Temporal range: Pliocene 

Scientific classification
Kingdom:  Animalia
Phylum:  Chordata
Class:  Mammalia
Order:  Primates
Suborder:  Haplorhini
Infraorder:  Simiiformes
Family:  Hominidae
Subfamily:  Homininae
Tribe:  Hominini
Genus:  †Australopithecus
Species:  Australopithecus anamensis  M.G. Leakey et al., 1995

Australopithecus anamensis is a hominin species that lived approximately four million years ago. Nearly one hundred fossil specimens are known from Kenya and Ethiopia, representing over 20 individuals. It is accepted that A. anamensis is ancestral to A. afarensis and continued an evolving lineage. Fossil evidence determines that Australopithecus anamensis is the earliest hominin species in the Turkana Basin.

The first fossilized specimen of the species, though not recognized as such at the time, was a single fragment of humerus (arm bone) found in Pliocene strata in the Kanapoi region of West Lake Turkana by a Harvard University research team in 1965. Bryan Patterson and William W. Howells's initial paper on the bone was published in Science in 1967; their initial analysis suggested an Australopithecus specimen and an age of 2.5 million years. Patterson and colleagues subsequently revised their estimation of the specimen's age to 4.0–4.5 mya based on faunal correlation data.
In 1994, the London-born Kenyan paleoanthropologist Meave Leakey and archaeologist Alan Walker excavated the Allia Bay site and uncovered several additional fragments of the hominid, including one complete lower jaw bone which closely resembles that of a Common Chimpanzee (Pan troglodytes) but whose teeth bear a greater resemblance to those of a Human. Based on the limited postcranial evidence available, A. anamensis appears to have been habitually bipedal, although it retained some primitive features of its upper limbs.
In 1995, Meave Leakey and her associates, taking note of differences between Australopithecus afarensis and the new finds, assigned them to a new species, A. anamensis, deriving its name from the Turkana word anam, meaning "lake". Leakey determined that this species was independent of many others.
Although the excavation team did not find hips, feet or legs, Meave Leakey believes that Australopithecus anamensis often climbed trees. Tree climbing was one behavior retained by early hominins until the appearance of the first Homo species about 2.5 million years ago. A. anamensis shares many traits with Australopithecus afarensis and may well be its direct predecessor. Fossil records for A. anamensis have been dated to between 4.2 and 3.9 million years ago, with recent findings from stratigraphic sequences dating to about 4.1–4.2 million years ago. Specimens have been found between two layers of volcanic ash, dated to 4.17 and 4.12 million years, coincidentally when A. afarensis appears in the fossil record.
The fossils (twenty one in total) include upper and lower jaws, cranial fragments, and the upper and lower parts of a leg bone (tibia). In addition to this, the aforementioned fragment of humerus found thirty years ago at the same site at Kanapoi has now been assigned to this species.
In 2006, a new A. anamensis find was officially announced, extending the range of A. anamensis into north east Ethiopia. Specifically, one site known as Asa Issie provided 30 A. anamensis fossils. These new fossils, sampled from a woodland context, include the largest hominid canine tooth yet recovered and the earliest Australopithecus femur. The find was in an area known as Middle Awash, home to several other more modern Australopithecus finds and only six miles (9.7 kilometers) away from the discovery site of Ardipithecus ramidus, the most modern species of Ardipithecus yet discovered. Ardipithecus was a more primitive hominid, considered the next known step below Australopithecus on the evolutionary tree. The A. anamensis find is dated to about 4.2 million years ago, the Ar. ramidus find to 4.4 million years ago, placing only 200,000 years between the two species and filling in yet another blank in the pre-Australopithecus hominid evolutionary timeline.

Australopithecus anamensis was found in Kenya, specifically at Allia Bay, East Turkana. Through analysis of stable isotope data, it is believed that their environment had more closed woodland canopies surrounding Lake Turkana than are present today. The greatest density of woodlands at Allia Bay was along the ancestral Omo River. There was believed to be more open savanna in the basin margins or uplands. Similarly at Allia Bay, it is suggested that the environment was much wetter. While it is not definitive, it also could have been possible that nut or seed-bearing trees could have been present at Allia Bay, however more research is needed.

Studies of the microwear on Australopithecus anamensis molar fossils show a pattern of long striations. This pattern is similar to the microwear on the molars of gorillas; suggesting that Australopithecus anamensis had a similar diet to that of the modern gorilla. The microwear patterns are consistent on all Australopithecus anamensis molar fossils regardless of location or time. This shows that their diet largely remained the same no matter what their environment.
The earliest dietary isotope evidence in Turkana Basin hominin species comes from the Australopithecus anamensis. This evidence suggests that their diet consisted primarily of C3 resources, possibly however with a small amount of C4 derived resources. Within the next 1.99- to 1.67-Ma time period, at least two distinctive hominin taxa shifted to a higher level of C4 resource consumption. At this point, there is no known cause for this shift in diet.

Oldest axial fossils discovered for the genus Australopithecus

by Chaffey College

[Image: oldestaxialf.jpg]
One of the fossils is an axis, or second cervical vertebra (C2) shown here in (a) ventral view, (b) dorsal view, © lateral view, and D) ventral view in articulation with modern H. sapiens atlas (C1). Note the correspondence between the Assa Issie axis and human atlas. Credit: Meyer & Williams, 2019

Scientists have published an article describing the oldest axial fossils yet discovered for the genus Australopithecus. Dated 4.2 million years ago, these and other fossils recovered from the Assa Issie site in the Middle Awash extend the known range of A. anamensis into northeastern Ethiopia. The fossils from the Assa Issie are extremely fragmentary, but each represents an important element previously unknown for the species Australopithecus anamensis.
In an upcoming article in the Journal of Human Evolution, paleoanthropologists Dr. Marc Meyer of Chaffey College and Scott Williams of New York University describe tell-tale signs that these early hominins had already evolved a human-like posture of the head and neck. "The bilobated facets of the first cervical vertebra are something we don't see in the great apes, but in humans is thought to provide a passive locking mechanism that keeps the head stable in erect posture," explains Meyer.
The scientists also point to the lack the pronounced retroglenoid tubercle of the great apes on two of the atlas (C1) fossils that indicate that like humans, anamensis lacked the atlantoclavicularis muscle, which would have reduced their capacity for climbing relative to the great apes—something scientists did not know until now.
Other features of the non-human ape spine are also absent in the hominin fossils, such as the ponticulus posticus, the bony form of a membrane in apes that protects the vertebral artery from being crushed when the head is cantilevered in front of the spine. The scientists report "lack of this feature in anamensis is consistent with a humanlike posture where the head is more centered above the spine".
The spinal column reveals other surprisingly human characters, such as an enlarged epiphyseal surface area that is a hallmark feature of bipedalism, as it improves the ability to resist the increased load magnitudes of upright posture. "Such a feature would also provide energy return during bipedal locomotion from the intervertebral discs in the form of elastic strain energy with rotary spinal movement" explain the scientists.
Finally, despite their great antiquity, like humans, the A. anamensis fossils from Issie exhibit an enlarged spinal canal compared to the apes. "This would confer an increase in the neurovascular contents of the canal, including the motor pool in the ventral horn of the australopith spinal cord well before the advent of genus Homo", says Meyer. The enlarged spinal canal provides the earliest evidence for an enlarged spinal cord in the hominin lineage and imparts significant neurological and vascular benefits for bipedal locomotion, and shatters the notion that spinal cord size in early hominins was small and apelike. This was another surprise, say the scientists, and provides evidence that a human-sized spinal cord evolved well before human brain size.

Journal Reference:

Marc R. Meyer and Scott A. Williams.  Earliest axial fossils from the genus Australopithecus. Journal of Human Evolution Volume 132, July 2019, Pages 189-214.

Australopitheus anamensis fossils demonstrate that craniodentally and postcranially the taxon was more primitive than its evolutionary successor Australopithecus afarensis. Postcranial evidence suggests habitual bipedality combined with primitive upper limbs and an inferred significant arboreal adaptation. Here we report on A. anamensis fossils from the Assa Issie locality in Ethiopia's Middle Awash area dated to ∼4.2 Ma, constituting the oldest known Australopithecus axial remains. Because the spine is the interface between major body segments, these fossils can be informative on the adaptation, behavior and our evolutionary understanding of A. anamensis. The atlas, or first cervical vertebra (C1), is similar in size to Homo sapiens, with synapomorphies in the articular facets and transverse processes. Absence of a retroglenoid tubercle suggests that, like humans, A. anamensis lacked the atlantoclavicularis muscle, resulting in reduced capacity for climbing relative to the great apes. The retroflexed C2 odontoid process and long C6 spinous process are reciprocates of facial prognathism, a long clivus and retroflexed foramen magnum, rather than indications of locomotor or postural behaviors. The T1 is derived in shape and size as in Homo with an enlarged vertebral body epiphyseal surfaces for mitigating the high-magnitude compressive loads of full-time bipedality. The full costal facet is unlike the extant great ape demifacet pattern and represents the oldest evidence for the derived univertebral pattern in hominins. These fossils augment other lines of evidence in A. anamensis indicating habitual bipedality despite some plesiomorphic vertebral traits related to craniofacial morphology independent of locomotor or postural behaviors (i.e., a long clivus and a retroflexed foramen magnum). Yet in contrast to craniodental lines of evidence, some aspects of vertebral morphology in A. anamensis appear more derived than its descendant A. afarensis.
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