| Classification of early hominid (Australopithecus) species: There seem to be two sets of Australopithecus species in the fossil record--the gracile (small) set and the robust (large) set. 1. Gracile Contains species A. afarensis and A. africanus --A. afarensis: oldest of well-known hominids (4-3 mya). Shortest bodies (around 4 feet tall, around 60 pounds). Smallest brain (300-500 cc) (modern human brain is about 1200-1300 cc). Skull still contains bony crests. Teeth larger than modern humans but smaller than ape; still large canines. Very sexually dimorphic--males about 50% larger than females. From neck down--especially from waist down--much more human-like: curvature of spine suggests erectness, forearm shorter than ape, hip bone short and flared like human, big toe less flexible and separated than ape. Footprints at Laetoli suggest bipedal, striding walking. --A. africanus: 3-2.3 mya. About the same body size as afarensis. Smaller teeth and face than afarensis, and larger brain (425->500 cc). Also bipedal. 2. Robust Contains species A. robustus and A. boisei --A. robustus: 1.8-1 mya. General characteristics similar to other Australopithecines, but larger--4.5-5 feet tall, ~ 100 pounds. Brain ~530 cc, with sagittal crest. Bones thick, suggesting heavy muscles. --A. boisei (first discovered by Louis Leakey 1959, interpreted to be tool-making human [Zinjanthropus], but really very large Australopithecus). Age range 2.5-1.3 mya. Similar to A. robustus, but enormous jaw and molars, heavy cheek bone and sagittal crest (adapted to very strong face muscles and very heavy chewing). 3. Recently-discovered older Australopithecines 1965 and 1995, fossils of even more primitive hominid discovered: A. anamensis. 4.2-3.9 mya. More ape-like jaw, large canine teeth, more V-shaped palate. But still bipedal. 1994, even older fossils (4.4 mya): at first, was classified A. ramidus, but has recently been interpreted to be a completely different genus or family of hominids, named Ardipithecus--full name is Ardipithecus ramidus. Even more ape-like, but no lower-body fossils found so far, so we do not know about its locomotion. Interpretation and Analysis: Gracile A.'s are older but in some ways more human-like. Robusts are newer but in some ways less human-like. MORE RECENT DOES NOT NECESSARILY MEAN MORE EVOLVED OR CLOSER TO HUMANS IN DEVELOPMENT OR ANCESTRY. Gracile A.'s seem to disappear about 2 mya (about the same time Homo appears ?!). Robust A.'s live on until about 1 mya, so are co-temporaneous with early Homo. Discovery of A. anamensis and Ar. ramidus pushes back hominid/ape divergence to before 5 mya. No strong evidence that Australopithecines ever made stone tools (although if some apes can use/make tools, A.'s probably did). For our purposes, Australopithecus begins when hominids start to walk erect. Homo begins when hominids start to make stone (and other) tools. Think about why hominids would have started to walk on two feet (pp. 166-172). Probably multiple contributing factors: Bipedalism is always slower than quadrapedalism. Appearance of bipedalism is not altogether a good thing (more lower back problems, more leg/knee/foot injuries, more intestinal/rectal/circulation problems [pressure on lower body from gravity])--HOMINIDS NOT ESPECIALLY "WELL-DESIGNED" FOR BIPEDAL WALKING. However, must have conferred some advantage--better ability to hunt? better ability to use tools? better ability to transport resources? better ability to see? better ability to use hands in ways that promote brain growth? |