14 DAY TRIAL // The evolution from ape to man is a complicated one, as there was not a direct line from our earliest ancestor to us, but a branched one, with many dead ramifications. Various families of primates differentiated 25-28 MA years ago (thus the oldest apes have this age) and the genera differentiation of the primates is no older than 7-11 MA. In fact, during our evolution, at a certain moment, the Earth was populated by more than one hominid at a time. It appears that the stage was left just for us about 30,000 years ago, when the last Neanderthals disappeared in Europe, even if some Homo erectus could have survived for a long time in the island of the Southeast Asia.
Here is a chronology of species discovered by paleontologists in our ancestry and not belonging to our genus, Homo:
1. Sahelanthropus chadensis lived 7 million years ago in a forested area, sharing its habitat with other monkeys and apes. It probably spent some time in the trees and perhaps walked upright.
The big surprise was the fact that it was found in 2001 in an area out of the East African Rift Valley, where the greatest discoveries of the last 35 years in the human evolution have been made, in the Sahel semidesert, in Chad, 2,500 km (1,550 mi) to the east.
Sahelanthropus shows that the first human ancestors did not live in a savanna, but in a forested area. It presented a mixture of human and ape traits.
The skull and the brain (320-380 cubic centimeters) were not larger than those of a chimp, but the short, flattened face, pronounced superciliary Arch, the less pushed forward mouth and jaws (than in the case of the apes) and the smaller, shorter canines, with a worn down tip, show that the creature was not a chimp. The molars are larger than in chimps, but smaller than in early humans, showing it ate less fruit.
The foramen magnum, the aperture at the base of the skull, where the spine cord connects to the brain, has an oval, not round shape like in chimps. Its shape and location points out that Toumai could walk in an upright position. Based on the skull's size, researchers believe Sahelanthropus was the size of a chimp.
Because of the modern look of Toumai, this means that Australopithecus is not our ancestor, but a lateral dead branch in our genealogical tree, like the Neanderthals. In this case Sahelanthropus would have evolved into an intermediary species, still undiscovered, whose descendants would have given birth to Homo rudolfensis (2.5 to 1.8 million years ago) and Homo habilis (1.9-1.5 million years ago), one of which was the first species of Homo genus.
The molecular clock shows that humans and chimps split some 4-5 million years ago. In this case, Sahelanthropus would be a common ancestor for humans and chimps. It could also be anatomically related to both humans and chimpanzees, but the ancestor of neither. Others say that it could be a female belonging to an ancestor of the gorillas. No matter the reality, Toumai shows that the last common ancestor of humans and chimpanzees did not look much like a chimp, as previously proposed, and chimps could have evolved many of their traits after splitting from humans.
2. Orrorin tugenensis was discovered in the Tugen Hills of Kenya and could have lived between 6.1 and 5.8 million years ago. It is one of the earliest fossils with evidence of bipedal locomotion in human lineage. Remains from at least five individuals have been found and this species could have lived in a habitat similar to that of the Sahelanthropus. A femur points to upright walk; a thick right humerus suggests tree-climbing skills but not brachiation. The full molars and small canines point to a human-like way of feeding, mostly fruits and vegetables and occasionally meat. The species, like Sahelanthropus, was about the size of a chimp.
Orrorin further proves that Australopithecus was a dead branch of hominids, as even if it is older, it is more human-like than the species of Australopithecus.
3. Ardipithecus lived between 5.8 to 5.2 million years ago. Two species have been found, Ardipithecus ramidus and Ardipithecus kadabba. Ardipithecus kadabba is dated to have lived between 5.8 million and 5.2 million years ago, while Ardipithecus ramidus 4.4-4.3 million years ago.
The canine teeth are still large, but the rest of the teeth are similar to Sahelanthropus and Ardipithecus kadabba is supposed to be the earliest species yet identified that evolved after the split from the lineage that led to chimps.
Ardipithecus, which was about the size of a chimp, walked upright, based on foot-bone morphology and this challenges current theories of the origins of bipedalism: as Ardipithecus, too, lived in forests rather than on the savanna.
Ardipithecus has some traits in common with the African great apes (chimps and gorillas), that's why some think it is an ancestral chimp rather than belonging to the human branch, but most consider it a proto-human because of the similarity in teeth with Australopithecus.
4. Australopithecus anamensis is the oldest Australopithecus, 4.1 to 3.9 million years old, found near Lake Turkana and Ethiopia. It is believed that Australopithecus anamensis often climbed trees, a behavior abandoned only by the genus Homo. Australopithecus anamensis could be the ancestor of Australopithecus afarensis and may as well be its direct predecessor. It lived in woodlands and had the largest hominid canines.
5. Australopithecus afarensis lived between 3.9 and 2.9 million years ago. It is the species of the famous Lucy and lived in Eastern Africa.
Australopithecus afarensis has smaller canines and molars than the apes, but still relatively larger than in modern humans, a small brain size (~380-430cm?) and a projecting face.
Australopithecus afarensis was at least partly arboreal. The anatomy of the hands, feet and shoulder joint in many ways favor this interpretation. The curvature of the finger and toe bones, the shoulder joint, the long arms and the ear labyrinth are similar to that of the apes. The presence of a wrist-locking mechanism might suggest that they were knuckle-walkers. But there are also traits of bipedalism: the pelvis, the femur and the ankle joint are far more human-like than ape-like. Big toes made it difficult if not impossible to grasp branches with the hindlimbs.
Males were much larger than females, thus Australopithecus afarensis could have lived like gorillas in small family groups with a single dominant male and several breeding females.
Australopithecus bahrelghazali, found in Chad, lived some 3.5 to 3 million years ago. It could be in fact a subspecies of Australopithecus afarensis and it is the sole Australopithecus found outside eastern/south Africa.
6. Australopithecus africanus lived between 3.3 and 2.4 million years ago, but was slender than Australopithecus afarensis, or gracile, and was much more like modern humans, with a larger brain and more humanoid facial features. Australopithecus africanus has been discovered in only four places in southern Africa.
Its arms were slightly larger than the legs (like in chimps). It had ape-like curved fingers for tree climbing. Many believe this species evolved into Paranthropus. Paranthropus robustus and Australopithecus africanus skulls are very alike despite the more heavily built features of Paranthropus robustus, adaptations for heavy chewing like a gorilla.
Both their brains measure about 400-500 cc and probably had an ape-like intelligence. Australopithecus africanus was slightly better adapted for bipedalism than Australopithecus afarensis and Homo-like post cranial features in comparison to Australopithecus afarensis make many believe that it evolved into the genus Homo.
7. Australopithecus garhi is a gracile species that lived 3-2 million years ago. It was found in Ethiopia's Afar Depression. The cranial capacity of Australopithecus garhi is just of 450 cc, but it has been proposed that Australopithecus garhi is the ancestor of the genus Homo, through a rapid evolution of just 200,000 and 300,000 years.
This is the only Australopithecus species for which stone tools have been found along with fossils and the first hominid to have processed tools. It could have used tools for cutting off flesh. In Bouri, Ethiopia, about 3,000 stone artifacts, 2.5 million years old, have been discovered.
8. Paranthropus was a bipedal hominid, called robust Australopithecus that evolved from the gracile forms. Paranthropus aethiopicus (2.7 to 2.5 million years old) lived in present-day Kenya. The brain was 40 % that of a human. The sex difference in size was almost double. Paranthropus was more vegetarian than the Australopithecus. They heavily specialized in chewing plants, like modern gorillas, with molars 4 times bigger than in humans, large chick bones and sagittal ridge. The smaller Paranthropus aethiopicus gave rise to Paranthropus boisei and Paranthropus robustus.
9. Paranthropus boisei (2.6 to 1.2 million years old) lived in Eastern Africa. They had a skull of about 500-550 cm?, not much larger than Australopithecus afarensis and Australopithecus africanus. Paranthropus boisei lived in dry savanna grasslands and woodland territories.
Males weighed 68 kg (150 lb) and were 4 feet 3 inches (1.3 m) tall, while females weighed 45 kg (100 lb) and stood 3 feet 5 inches (1.05 m) tall. This is the greatest sexual dimorphism recorded in a hominid and they were the largest ape-men.
10. Paranthropus robustus (2 to 1.2 million years ago) lived in South Africa. Males were 4 feet (1.2 m) tall and weighed 54 kg (120 lb), as big as a chimp. The average brain size was 410 and 530 cc, as in a large chimp. They lived on vegetable diets (roots, stems and seeds) in open woodland and savanna. The jaws, premolars and molars were larger than in chimps.