Evolution of Hominins

Evolution of the Hominins

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: TheAdapidae are lemurlike and the Omomyidae aretarsierlike.

Evolution of Hominins

The Early Hominins: Bipedal Primates

Tribe Hominini: the bipedal primates (thehominins)

Genus Australopithecus: small-brained, gracile (slender)hominins with a mixed vegetable/fruit diet

Genus Paranthropus: small-brained, robust hominins witha grassland vegetable diet

Genus Homo: large-brained, omnivorous hominins

definitions

Foramen Magnum

The hole in the base of the skull through which thespinal cord emerges and around the outside ofwhich the top vertebra articulates.

Prognathism

The jutting forward of the lower face and jawarea.

Sagittal Crest

A ridge of bone, running from front to back alongthe top of the skull, for the attachment of chewingmuscles.

Evolution of the Hominins

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Evolution of the Hominins

Bipedalism

What’s the benefit of standing upright? Whenwe stand and walk, with our trunk erect andknees straight, we have to balance our bodiesvertically on two relatively small points ofcontact with the ground.

We can’t run particularly fast, and we aren’tvery 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 inlocomotion would have meant that our ancestorscould transport food from open areas to saferlocations.

Vigilance Model

By elevating the head, bipedalism helped ourancestors locate potential sources of food anddanger.

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Fig. 6-3, p. 130

Ardipithecus ramidus

An Ethiopian member of a research team led bypaleoanthropologist Tim White discovered the earliestknown australopith species in Ethiopia in 1994. Theserecognizably human fossils were estimated to be about 4.4million years old. White and his colleagues gave theirdiscovery the name Ardipithecus ramidus. Ramid means"root" in the Afar language of Ethiopia, and refers to thecloseness of this new species to the roots of humanity. Atthe time of this discovery, the genus Australopithecus wasscientifically well established. White devised the genusname Ardipithecus to distinguish this new species fromother australopiths because it had a very ancientcombination of apelike and humanlike traits.

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Evolution of the Hominins

More Australopithecines

Although little changed from Australopithecusafarensis, the fossils representing the next period arestill called by their original name, A. africanus.

The remains of this species have been found mostly inSouth Africa.

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

There are a few differences: their faces are a bit lessprognathous and they lack a sagittal crest.

Evolution of the Hominins

Still More Hominins

Between 3 and 2 mya, two new types of homininsappear in the fossil record, Paranthropus and ourgenus Homo.

Paranthropus retains the chimpanzee-sized brains andsmall bodies of Australopithecus but has evolved anotable robusticity in the areas of the skull involvedwith chewing.

The second new hominin genus that appearedabout 2.5 mya is the one to which modern humansbelong, Homo.

Evolution of the Hominins

Putting It All Together

Increased environmental variability resulting in a seriesof newly emerging, complex, and diverse habitats mayhave initially promoted different adaptations amonghominin populations.

Unable to survive well enough to perpetuate themselvesin the face of decreasing resources these now-extincthominins were possibly outcompeted for space andresources by the better adapted.

In this case, the adaptive response that included the abilityfor endurance running and an increase in brain size provedsuccessful in the long run.

Australopithecus anamensis

In 1965 a research team form Harvard University discovered asingle arm bone of an early human at the site of Kanapoi innorthern Kenya. The researchers estimated this bone to be 4million years old, but could not identify the species to which itbelonged. It was not until 1994 that a research team, led bypaleoanthropologist Meave Leakey, found numerous teeth andfragments of bone at the site that could be linked to thepreviously discovered fossil. Leakey and her colleaguesdetermined that the fossils were those of a very primitive speciesof australopith, which was given the name Australopithecusanamensis. Researchers have since found other A. anamensisfossils at nearby sites, dating between about 4.2 million and 3.9million years old. The skull of this species appears apelike, whileits enlarged tibia or lower leg bone, indicates that it supported itsfull body weight on one leg at a time, as in regular bipedalwalking

Australopithecus afarensis

Australopithecus anamensis was quite similar to another, muchbetter-known species, A. afarensis, a gracile australopith thatthrived in eastern Africa between about 3.9 million and 3 millionyears ago. The most celebrated fossil of this species, known asLucy, is a partial skeleton of a female discovered bypaleoanthropologist Donald Johanson in 1974 at Hadar, Ethiopia.Lucy lived 3.2 million years ago. Several hundred fossils of thisspecies have been described from Hadar, including a collectionrepresenting at least 13 individuals of both sexes and variousages, all from a single site that is dated 3.2 million years old.

Researchers working in northern Tanzania have also foundfossilized bones of A. afarensis at Laetoli, a 3.6 million year oldsite best known for spectacular trails of bipedal human footprints(and the prints of other animals) preserved in a hardened volcanicash. These footprints were discovered in 1978 by a research teamled by paleoanthropologist Mary Leakey. They provide irrefutableevidence that australopiths regularly walked bipedally.

LUCY

The controversy about how the australopiths moved has mainlyfocused on Lucy's species A. afarensis. While Lucy certainlywalked upright, she stood only 3.5 feet tall and had longer, morepowerful arms than most later human species, which suggeststhat she was also adept at climbing trees. And while the Laetolifootprints were made by bipedal humans, some scientists haveargued that the imprints of the heel, arch, and toes are notexactly like those made by modern human feet. In addition, otherfossils from Hadar and Laetoli come from individuals much largerthan Lucy, up to 5 feet tall. This has caused controversy overwhether the entire set of fossils represents one or two species,although most scientists accept the single-species idea sincelarge and small adults, probably male and female, occurredtogether at the same site at Hadar.

Australopithecus africanus

Australopithecus africanus thrived in what is now theTransvaal region of South Africa between about 3.5million and 2.5 million years ago. The anatomistRaymond Dart described this species -- the firstknown australopith -- on the basis of a fossildiscovered in 1924 at Taung, South Africa. For twodecades after this discovery, almost no one in thescientific community believed Dart's claim that theskull came from an ancestral human. In the late1930s and 1940s, teams led by paleontologist RobertBroom unearthed many more australopith skulls andother bones from the Transvaal sites of Sterkfonteinand Swartkrans.

Australopithecus Afarensis

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lucy

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Paranthropus

Paranthropus aethiopicus

The earliest known robust species, Paranthropus aethiopicus, had evolved in eastern Africa by 2.7million years ago. In 1985 at West Turkana, Kenya, paleoanthropologist Alan Walker discoveredthe fossil skull that defined this species.

Paranthropus boisei

Paranthropus boisei, the other well-known East African robust australopith, lived over a largegeographic range between about 2.3 million and 1.2 million years ago. In 1959 Mary Leakeydiscovered the first fossil of this species -- a nearly complete skull at the site of Olduvai Gorge inTanzania. Paleoanthropologist Louis Leakey, husband of Mary, named the new speciesZinjanthropus boisei (Zinjanthropus translates as "East African man"). This skull, which is dated to1.8 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 molarsare four times the size of those in modern humans. Since the discovery of Zinjanthropus, nowrecognized 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, livedbetween about 1.8 million and 1.3 million years ago in the Transvaal, the same region that washome to A. africanus

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Evolution of the Hominins

Summary

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

This successful group has left fossils all overAfrica, Europe, and Asia.

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