Evolution of the Hominins

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The Origin and Evolution of the Primates There are large gaps in the primate fossil

record. Still we can put together a general, if tentative, picture of the course of primate evolution.

Undisputed primates appear about 55 mya.

The early primates come in two groups: The Adapidae are lemurlike and the Omomyidae are tarsierlike.

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Evolution of Hominins

The Early Hominins: Bipedal Primates Tribe Hominini: the bipedal primates

(the hominins) Genus Australopithecus: small-brained,

gracile (slender) hominins with a mixed vegetable/fruit diet

Genus Paranthropus: small-brained, robust hominins with a grassland vegetable diet

Genus Homo: large-brained, omnivorous hominins

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definitions

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Foramen Magnum The hole in the base of the skull through

which the spinal cord emerges and around the outside of which the top vertebra articulates.

Prognathism The jutting forward of the lower face and jaw

area. Sagittal Crest

A ridge of bone, running from front to back along the top of the skull, for the attachment of chewing muscles.

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Evolution of the Hominins

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Fig. 6-2, p. 130

Fig. 6-7, p. 136

Fig. 6-10, p. 138

Evolution of the Hominins Bipedalism

What’s the benefit of standing upright? When we stand and walk, with our trunk erect and knees straight, we have to balance our bodies vertically on two relatively small points of contact with the ground.

We can’t run particularly fast, and we aren’t very stable on rough or slippery surfaces.

Evolution of the Hominins Explaining the Evolution of

Bipedalism Carrying Model

Freeing the arms and hands from a role in locomotion would have meant that our ancestors could transport food from open areas to safer locations.

Vigilance Model By elevating the head, bipedalism

helped our ancestors locate potential sources of food and danger.

p. 135

Fig. 6-3, p. 130

Ardipithecus ramidus

An Ethiopian member of a research team led by paleoanthropologist Tim White discovered the earliest known australopith species in Ethiopia in 1994. These recognizably human fossils were estimated to be about 4.4 million years old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means "root" in the Afar language of Ethiopia, and refers to the closeness of this new species to the roots of humanity. At the time of this discovery, the genus Australopithecus was scientifically well established. White devised the genus name Ardipithecus to distinguish this new species from other australopiths because it had a very ancient combination of apelike and humanlike traits.

Evolution of the Hominins More Australopithecines

Although little changed from Australopithecus afarensis, the fossils representing the next period are still called by their original name, A. africanus.

The remains of this species have been found mostly in South Africa.

They have the same body size and shape and the same brain size as A. afarensis.

There are a few differences: their faces are a bit less prognathous and they lack a sagittal crest.

Evolution of the Hominins Still More Hominins

Between 3 and 2 mya, two new types of hominins appear in the fossil record, Paranthropus and our genus Homo.

Paranthropus retains the chimpanzee-sized brains and small bodies of Australopithecus but has evolved a notable robusticity in the areas of the skull involved with chewing. The second new hominin genus that

appeared about 2.5 mya is the one to which modern humans belong, Homo.

Evolution of the Hominins Putting It All Together

Increased environmental variability resulting in a series of newly emerging, complex, and diverse habitats may have initially promoted different adaptations among hominin populations.

Unable to survive well enough to perpetuate themselves in the face of decreasing resources these now-extinct hominins were possibly outcompeted for space and resources by the better adapted.

In this case, the adaptive response that included the ability for endurance running and an increase in brain size proved successful in the long run.

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Australopithecus anamensis In 1965 a research team form Harvard University

discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be 4 million years old, but could not identify the species to which it belonged. It was not until 1994 that a research team, led by paleoanthropologist Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of a very primitive species of australopith, which was given the name Australopithecus anamensis. Researchers have since found other A. anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, while its enlarged tibia or lower leg bone, indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking

Australopithecus afarensis Australopithecus anamensis was quite similar to another,

much better-known species, A. afarensis, a gracile australopith that thrived in eastern Africa between about 3.9 million and 3 million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by paleoanthropologist Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Several hundred fossils of this species have been described from Hadar, including a collection representing at least 13 individuals of both sexes and various ages, all from a single site that is dated 3.2 million years old.

Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli, a 3.6 million year old site best known for spectacular trails of bipedal human footprints (and the prints of other animals) preserved in a hardened volcanic ash. These footprints were discovered in 1978 by a research team led by paleoanthropologist Mary Leakey. They provide irrefutable evidence that australopiths regularly walked bipedally.

LUCY

The controversy about how the australopiths moved has mainly focused on Lucy's species A. afarensis. While Lucy certainly walked upright, she stood only 3.5 feet tall and had longer, more powerful arms than most later human species, which suggests that she was also adept at climbing trees. And while the Laetoli footprints were made by bipedal humans, some scientists have argued that the imprints of the heel, arch, and toes are not exactly like those made by modern human feet. In addition, other fossils from Hadar and Laetoli come from individuals much larger than Lucy, up to 5 feet tall. This has caused controversy over whether the entire set of fossils represents one or two species, although most scientists accept the single-species idea since large and small adults, probably male and female, occurred together at the same site at Hadar.

Australopithecus africanus Australopithecus africanus thrived in what is

now the Transvaal region of South Africa between about 3.5 million and 2.5 million years ago. The anatomist Raymond Dart described this species -- the first known australopith -- on the basis of a fossil discovered in 1924 at Taung, South Africa. For two decades after this discovery, almost no one in the scientific community believed Dart's claim that the skull came from an ancestral human. In the late 1930s and 1940s, teams led by paleontologist Robert Broom unearthed many more australopith skulls and other bones from the Transvaal sites of Sterkfontein and Swartkrans.

Australopithecus Afarensis

lucy

Paranthropus

Paranthropus aethiopicus The earliest known robust species, Paranthropus aethiopicus, had evolved in

eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, paleoanthropologist Alan Walker discovered the fossil skull that defined this species.

Paranthropus boisei Paranthropus boisei, the other well-known East African robust australopith,

lived over a large geographic range between about 2.3 million and 1.2 million years ago. In 1959 Mary Leakey discovered the first fossil of this species -- a nearly complete skull at the site of Olduvai Gorge in Tanzania. Paleoanthropologist Louis Leakey, husband of Mary, named the new species Zinjanthropus boisei (Zinjanthropus translates as "East African man"). This skull, which is dated to 1.8 million years ago, has the most specialized features of all the robust species. It has a massive, wide, and dished-in face that was capable of withstanding extreme chewing forces, and its molars are four times the size of those in modern humans. Since the discovery of Zinjanthropus, now recognized as an australopith, scientists have found great numbers of P. boisei fossils in Tanzania, Kenya, and Ethiopia.

Paranthropus robustus The southern robust species, which has the descriptive name Paranthropus

robustus, lived between about 1.8 million and 1.3 million years ago in the Transvaal, the same region that was home to A. africanus

Fig. 6-5, p. 133

p. 131

Fig. 6-12, p. 141

Evolution of the Hominins Summary

About 23 mya, the hominoids appear in the form of primitive apes.

This successful group has left fossils all over Africa, Europe, and Asia.

It is from one of the African apes that our tribe, Hominini, branched off 6 to 5 mya.