Early Hominids Chronology of Hominid Evolution,The Varied Australopithecines, Beginning of Homo and Tools

Chronology of Human Evolution If we compare Earth’s History to a 24 hour day: Earth originates at midnight. The earliest fossils were deposited at 5:45am The first vertebrates appeared at 9:02 pm Earliest mammals at 10:45pm Earliest primates at 11:43pm The earliest hominids (australopithecines) at 11:58pm Homo Sapiens arrives 36 seconds before midnight. For the study of Hominid evolution, the Pliocene(5 to 2 m.y.a), Pleistocene (2 m.y.a to 10,000 B.P.) and Recent (10,000 B.P. to present) epochs are most important. Until the end of the Pliocene, the main hominid was Australopithecus (Africa), by the start of the Pleistocene they evolved into Homo.

Pleistocene The Pleistocene is considered the epoch of human life, sub-divided into lower, middle and upper. Each subdivision is associated with a particular group of hominids. Late Australopithecus and early Homo lived during Lower. Homo Erectus spanned most of Middle, and Homo sapiens appeared late in the Middle and was the sole hominid of the Upper. During the 2nd million years of this era, there were several ice ages, or glacials, major advances of continental ice sheets in Europe and N. America. These periods were separated by interglacials, long warm periods between major glacials. During interglacials, the climate warmed up forests returned to the areas. Hominid fossils found in association with animals known to occur in cold and warm climates.

The varied Australopithecines We still don’t know the identity of our Miocene ancestors, we do know that they evolved into a varied group of Pliocene-Pleistocene hominids known as the australopithecines. Today the distinction between the australopitecines and later hominids is made at the genus level. Most significant fossils in this genus were found East Africa(Table 6.1) Ardipithecus ramidus is recognized as the earliest known hominid, which evolved into A. anamensis, a bipedal hominid, which evolved into A. afarensis, which is considered ancestral to all the later australopithecines as well as to homo.

Species of Australopithecus A. amamensis (4.2 m.y.a) A. afarensis (3.8? To 3.0 m.y.a) A. africanus (3.0 to 2.5 m.y.a) A. robustus (2.6 to 2.0 m.y.a) A. boisei (2.6 to 1.2 m.y.a) Homo appeared around 2 m.y.a.

Australopithecus afarensis Earliest hominid species. Fossils found in northern Tanzania and parts of Ethiopia. “Lucy” was one of these fossil finds. Similar in many ways to chimps and gorillas that are common ancestry with African apes must be very recent (no more than 8 m.y.a)

Dentition of A. afarensis Like apes, unlike modern humans, A. afarensis had sharp canine teeth that projected beyond the other teeth. Lower premolar was pointed and projecting to sharpen the upper canine. It had one long cusp and one tiny bump that is like a bicuspid premolar that eventually developed in hominid evolution. Evidence shows powerful chewing associated with savanna vegitation was entering their feeding pattern. Massive back teeth, jaws, and facial and cranial structures suggest a diet demanding extensive grinding and powerful crushing See pg 173 Fig 7.2

What makes us Human? Bipedal locomotion, big brains, childhood dependency, use of language and tools? We can be defined from features we have lost: big back teeth. Bipedalism has characterized human evolution since it split from African Apes. Skull of A afarensis contrasts with those of later hominids: brain case is very small. Very similar to a chimp.

Bipedalism Bipedalism-upright two-legged locomotion-is the key feature differentiating earlier hominids from the apes. May be more than 5 m.y.a The structure of the hip, pelvic, leg and foot bones confirm that upright bipedalism was A. afarensis’s mode of locomotion. Actually started with A. anamensis (4.2 m.y.a) Considered an adaptation to open grassland. Preceded tool manufacture and expansion of hominid brain. Early hominids also preserved enough of their ape-like anatomy to take to the trees to sleep at night and keep away from predators

Advantages of Bipedalism Ability to see over long grass Carrying items back to home base Reducing bodies exposure to sunlight

Why did we adapt to bipedalism? Change in climate: to cooler and drier, shrinking the aboreal habitat. Deepening of Rift Valley-caused split environment which may have caused the split in hominoids and chimps. (174) To protect brains from overheating. Quadrupedalism exposes body to 60% more UV radiation. Upright body can catch cooler breeze above ground.

Sexual Dimorphism in A. afarensis Difference in jaw size of male and female Females stood between 3 and 4 feet, whereas males reached 5 feet. Males weighted twice as much as females Both males and females had more robust muscles and bones than modern humans. Mixture of apelike and hominoid features in both sexes

How we walk Hominids: alternating swing and stance for each foot. Old World Monkeys (not humans): always supported by 2 limbs. Bipeds-are supported by one limb at a time. Homo pelvises are much smaller than Australopithecine, which shows an adaptation to bipedalism. Spines were similar but not identical to ours. Expansion of the birth canal is a trend in hominid evolution, due to our larger brain.

Early Homo Between 3 and 2 m.y.a, ancestors of Homo split off and became reproductively isolated from the later australopithecines that coexisted with Homo until 1.2 m.y.a . 1.7 m.y.a there were two hominid groups. 1) Homo erectus- had a larger brain which had an increase in areas of the brain that regulate higher mental functions. These were our ancestors-they hunted and gathered, made sophisticated tools and soon displaced A. boisei.

Separation between A. boisei and early Homo One theory proposes that A. afarensis split into 2 groups. One group, the ancestors of Homo, became reproductively isolated from other hominids between 3 and 3 m.y.a. This group became Homo habilis (2-1.7 m.y.a) which evolved to H.erectus. The other groups of A. afarensis evolved into the various kinds of australopithecines.

H. rudolfensis and H. habilis A skull found in 1972 –unusual combination of a large brain and very large molars. Created much controversy and debate. (183) In 1986-it was named H. rudolfensis. Some think rudolfensis lived earlier than and is an ancestor to habilis. Some think that rudofensis and habilis are male and female members of the same species-habilis. Some think they are separate, but coexisted. And others think that one of them gave rise to H. erectus.

Tools Tool making may have had something to do with the split between A. boisei and Homo. Oldowan pebble tools: are pieces of stone about the size of a tennis ball. Flakes were struck off both sides to form a cutting edge. Early homo also used tools that they did not make: naturally chipped or cracked objects.

A. Garhi and Early Stone Tools 1999 new species of hominid discovered along with earliest traces of butchery. Fossils date to 2.5 m.y.a May be the link between Australopithecus and Homo. Species named A. Garhi “surprise”. Important for 3 reasons: 1) new potential ancestor to humans. 2) show that the femur had elongated by 2.5 m.y.a., which was a million years before the forearm shortened. 3) evidence that large animals were being butchered shows early stone technology was aimed at getting meat and marrow from big game = dietary revolution.

A little more on tools Tools show that Australopithecines were toolmakers with the capacity for culture. Australopithecines evolved to Homo, which became the most efficient exploiter of the savanna niche. H. erectus eventually expanded into Asia and Europe. Next: Modern Humans!!!