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Australopithecus from Latin australis 'southern', and Greek (pithekos)
'ape'; singular: australopith) is a genus of early hominins that existed in
Africa during the Late Pliocene and Early Pleistocene. The genera Homo (which
includes modern humans), Paranthropus, and Kenyanthropus evolved from
Australopithecus. Australopithecus is a member of the subtribe
Australopithecina, which also includes Ardipithecus, though the term
"australopithecine" is sometimes used to refer only to members of
Australopithecus. Species include A. garhi, A. africanus, A. sediba, A.
afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as
to whether some Australopithecus species should be reclassified into new
genera, or if Paranthropus and Kenyanthropus are synonymous with
Australopithecus, in part because of the taxonomic inconsistency.
The earliest known member of the genus, A. anamensis, existed in eastern Africa
around 4.2 million years ago. Australopithecus fossils become more widely
dispersed throughout eastern and southern Africa (the Chadian A. bahrelghazali
indicates the genus was much more widespread than the fossil record suggests),
before eventually becoming extinct 1.9 million years ago (or 1.2 to 0.6 million
years ago if Paranthropus is included). While none of the groups normally
directly assigned to this group survived, Australopithecus gave rise to living
descendants, as the genus Homo emerged from an Australopithecus species at some
time between 3 and 2 million years ago. Australopithecus possessed two of three
duplicated genes derived from roughly 3.4 and 2.4 million years ago (SRGAP2B
and SRGAP2C), the second of which contributed to the increase in number and
migration of neurons in the human brain. Significant changes to the hand first
appear in the fossil record of later A. afarensis about 3 million years ago
(fingers shortened relative to thumb and changes to the joints between the
index finger and the trapezium and capitate).
Research history:
Main article: Taung Child:
The first Australopithecus specimen, the type specimen, was discovered in 1924
in a lime quarry by workers at Taung, South Africa. The specimen was studied by
the Australian anatomist Raymond Dart, who was then working at the University
of the Witwatersrand in Johannesburg. The fossil skull was from a
three-year-old bipedal primate that he named Australopithecus africanus. The
first report was published in Nature in February 1925. Dart realised
that the fossil contained a number of humanoid features, and so he came to the
conclusion that this was an early human ancestor. Later, Scottish
paleontologist Robert Broom and Dart set out to search for more early hominin
specimens, and several more A. africanus remains from various sites. Initially,
anthropologists were largely hostile to the idea that these discoveries were
anything but apes, though this changed during the late 1940s.
In 1950, evolutionary biologist Ernst Walter Mayr said that all bipedal apes
should be classified into the genus Homo, and considered renaming
Australopithecus to Homo transvaalensis. However, the contrary view taken by
Robinson in 1954, excluding australopiths from Homo, became the prevalent view.
The first australopithecine fossil discovered in eastern Africa was an A.
boisei skull excavated by Mary Leakey in 1959 in Olduvai Gorge, Tanzania. Since
then, the Leakey family has continued to excavate the gorge, uncovering further
evidence for australopithecines, as well as for Homo habilis and Homo erectus.
The scientific community took 20 more years to widely accept Australopithecus
as a member of the human family tree.
In 1997, an almost complete Australopithecus skeleton with skull was found in
the Sterkfontein caves of Gauteng, South Africa. It is now called "Little
Foot" and it is around 3.7 million years old. It was named
Australopithecus prometheus which has since been placed within A. africanus.
Other fossil remains found in the same cave in 2008 were named Australopithecus
sediba, which lived 1.9 million years ago. A. africanus probably evolved into
A. sediba, which some scientists think may have evolved into H. erectus, though
this is heavily disputed. A. afarensis, A. anamensis, and A. bahrelghazali were
split off into the genus Praeanthropus, but this genus has been largely
dismissed.
Classification:
The genus Australopithecus is considered to be a grade taxon, whose members are
united by their similar physiology rather than close relations with each other
over other hominin genera. It is considered the ancestor of Homo,
Kenyanthropus, and Paranthropus, and as such, is paraphyletic, not consisting
of a common ancestor and all of its descendants. Resolving this problem would
cause major ramifications in the nomenclature of all descendent species.
Possibilities suggested have been to rename Homo sapiens to Australopithecus
sapiens (or even Pan sapiens or to move some Australopithecus species into new
genera. Opinions differ as to whether the Paranthropus should be included
within Australopithecus, and Paranthropus is suggested along with Homo to have
developed as part of a clade with A. africanus as its basal root. The members
of Paranthropus appear to have a distinct robustness compared to the gracile
australopiths, but it is unclear if this indicates all members stemmed from a
common ancestor or independently evolved similar traits from occupying a
similar niche. In 2002 and again in 2007, Cele-Conde et al. suggested that A.
africanus be moved to Paranthropus On the basis of craniodental evidence,
Strait and Grine (2004) suggest that A. anamensis and A. garhi should be
assigned to new genera. It is debated whether or not A. bahrelghazali should be
considered simply a western variant of A. afarensis instead of a separate
species.
African hominin timeline (in mya) View references
Evolution:
Map of the fossil sites of the early australopithecines in Africa:
A. anamensis may have descended from or was closely related to Ardipithecus
ramidus. A. anamensis shows some similarities to both Ar. ramidus and
Sahelanthropus. Australopiths shared several traits with modern apes and
humans, and were widespread throughout Eastern and Northern Africa by 3.5
million years ago (MYA). The earliest evidence of fundamentally bipedal
hominins is a 3.6 MYA fossil trackway in Laetoli, Tanzania, which bears a
remarkable similarity to those of modern humans. The footprints have generally
been classified as australopith, as they are the only form of prehuman hominins
known to have existed in that region at that time. According to the Chimpanzee
Genome Project, the human–chimpanzee last common ancestor existed about
five to six million years ago, assuming a constant rate of mutation. However,
hominin species dated to earlier than the date could call this into question.
Sahelanthropus tchadensis, commonly called "Toumai", is about seven
million years old and Orrorin tugenensis lived at least six million years ago.
Since little is known of them, they remain controversial among scientists since
the molecular clock in humans has determined that humans and chimpanzees had a
genetic split at least a million years later. One theory suggests that the
human and chimpanzee lineages diverged somewhat at first, then some populations
interbred around one million years after diverging.
Anatomy:
The brains of most species of Australopithecus were roughly 35% of the size of
a modern human brain with an endocranial volume average of 466 cc (28.4 cu in).
Although this is more than the average endocranial volume of chimpanzee brains
at 360 cc (22 cu in) the earliest australopiths (A. anamensis) appear to have
been within the chimpanzee range, whereas some later australopith specimens
have a larger endocranial volume than that of some early Homo fossils.
Most species of Australopithecus were diminutive and gracile, usually standing
1.2 to 1.4 m (3 ft 11 in to 4 ft 7 in) tall. It is possible that they exhibited
a considerable degree of sexual dimorphism, males being larger than females. In
modern populations, males are on average a mere 15% larger than females, while
in Australopithecus, males could be up to 50% larger than females by some
estimates. However, the degree of sexual dimorphism is debated due to the
fragmentary nature of australopith remains. One paper finds that
Australopithecus Afarensis had a level of dimorphism close to modern humans.
According to A. Zihlman, Australopithecus body proportions closely resemble
those of bonobos (Pan paniscus), leading evolutionary biologist Jeremy Griffith
to suggest that bonobos may be phenotypically similar to Australopithecus.
Furthermore, thermoregulatory models suggest that australopiths were fully hair
covered, more like chimpanzees and bonobos, and unlike humans.
The fossil record seems to indicate that Australopithecus is ancestral to Homo
and modern humans. It was once assumed that large brain size had been a
precursor to bipedalism, but the discovery of Australopithecus with a small
brain but developed bipedality upset this theory. Nonetheless, it remains a
matter of controversy as to how bipedalism first emerged. The advantages of
bipedalism were that it left the hands free to grasp objects (e.g., carry food
and young), and allowed the eyes to look over tall grasses for possible food
sources or predators, but it is also argued that these advantages were not
significant enough to cause the emergence of bipedalism.
Earlier fossils, such as Orrorin tugenensis, indicate bipedalism around six
million years ago, around the time of the split between humans and chimpanzees
indicated by genetic studies. This suggests that erect, straight-legged walking
originated as an adaptation to tree-dwelling. Major changes to the pelvis and
feet had already taken place before Australopithecus. It was once thought that
humans descended from a knuckle-walking ancestor, but this is not
well-supported. Australopithecines have thirty two teeth, like modern humans.
Their molars were parallel, like those of great apes, and they had a slight
pre-canine gap (diastema). Their canines were smaller, like modern humans, and
with the teeth less interlocked than in previous hominins. In fact, in some
australopithecines, the canines are shaped more like incisors. The molars of
Australopithicus fit together in much the same way those of humans do, with low
crowns and four low, rounded cusps used for crushing. They have cutting edges
on the crests. However, australopiths generally evolved a larger postcanine
dentition with thicker enamel. Australopiths in general had thick enamel, like
Homo, while other great apes have markedly thinner enamel. Robust australopiths
wore their molar surfaces down flat, unlike the more gracile species, who kept
their crests.
Diet:
In a 1979 preliminary microwear study of Australopithecus fossil teeth,
anthropologist Alan Walker theorized that robust australopiths ate
predominantly fruit (frugivory). Australopithecus species are thought to have
eaten mainly fruit, vegetables, and tubers, and perhaps easy to catch animals
such as small lizards. Much research has focused on a comparison between the
South African species A. africanus and Paranthropus robustus. Early analyses of
dental microwear in these two species showed, compared to P. robustus, A.
africanus had fewer microwear features and more scratches as opposed to pits on
its molar wear facets. Microwear patterns on the cheek teeth of A. afarensis
and A. anamensis indicate that A. afarensis predominantly ate fruits and
leaves, whereas A. anamensis included grasses and seeds (in addition to fruits
and leaves). The thickening of enamel in australopiths may have been a response
to eating more ground-bound foods such as tubers, nuts, and cereal grains with
gritty dirt and other small particulates which would wear away enamel.
Gracile australopiths had larger incisors, which indicates tearing food was
important, perhaps eating scavenged meat. Nonetheless, the wearing patterns on
the teeth support a largely herbivorous diet. In 1992, trace-element studies of
the strontium/calcium ratios in robust australopith fossils suggested the
possibility of animal consumption, as they did in 1994 using stable carbon
isotopic analysis. In 2005, fossil animal bones with butchery marks dating to
2.6 million years old were found at the site of Gona, Ethiopia. This implies
meat consumption by at least one of three species of hominins occurring around
that time: A. africanus, A. garhi, and/or P. aethiopicus. In 2010, fossils of
butchered animal bones dated 3.4 million years old were found in Ethiopia,
close to regions where australopith fossils were found. Robust
australopithecines (Paranthropus) had larger cheek teeth than gracile
australopiths, possibly because robust australopithecines had more tough,
fibrous plant material in their diets, whereas gracile australopiths ate more
hard and brittle foods. However, such divergence in chewing adaptations may
instead have been a response to fallback food availability. In leaner times,
robust and gracile australopithecines may have turned to different low-quality
foods (fibrous plants for the former, and hard food for the latter), but in
more bountiful times, they had more variable and overlapping diets. A study in
2018 found non-carious cervical lesions, caused by acid erosion, on the teeth
of A. africanus, probably caused by consumption of acidic fruit.
Technology:
It was once thought that Australopithecus could not produce tools like Homo,
but the discovery of A. garhi associated with large mammal bones bearing
evidence of processing by stone tools showed this to not have been the case.
Discovered in 1994, this was the oldest evidence of manufacturing at the time
until the 2010 discovery of cut marks dating to 3.4 mya attributed to A.
afarensis, and the 2015 discovery of the Lomekwi culture from Lake Turkana
dating to 3.3 mya possibly attributed to Kenyanthropus. More stone tools dating
to about 2.6 mya in Ledi-Geraru in the Afar Region were found in 2019, though
these may be attributed to Homo.
Notable specimens KT-12/H1, an A. bahrelghazali mandibular fragment, discovered
1995 in Sahara, Chad AL 129-1, an A. afarensis knee joint, discovered 1973 in
Hadar, Ethiopia Karabo, a juvenile male A. sediba, discovered in South Africa
Laetoli footprints, preserved hominin footprints in Tanzania Lucy, a
40%-complete skeleton of a female A. afarensis, discovered 1974 in Hadar,
Ethiopia Selam, remains of a three-year-old A. afarensis female, discovered in
Dikika, Ethiopia STS 5 (Mrs. Ples), the most complete skull of an A. africanus
ever found in South Africa STS 14, remains of an A. africanus, discovered 1947
in Sterkfontein, South Africa STS 71, skull of an A. africanus, discovered 1947
in Sterkfontein, South Africa Taung Child, skull of a young A. africanus,
discovered 1924 in Taung, South Africa
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