Brief description of the last 23mya of evolution leading to modern humans
Around 23mya conditions in Africa became hospitable for a new type of primate. Apes diversified greatly in Africa and occupied many niches. Sizes, diets and habitats had a wider range as well. The adaptation to these different niches brought about many traits at least some of which survive today in apes and humans. There were up to 100 species of ape in this epoch compared to 5 today (chimpanzee, bonobo, orangutans, gorillas and gibbons). Those African apes spread across the new world finding themselves in Europe and Asia. In tracing our lineage through hominoids it would be a good idea to keen in on primitive traits we have and to identify which previous species share them. some of these traits are bipedalism, encephalization, suspensory hanging and exaggerated secondary sexual characteristics. Other important considerations are the habitats in which these traits developed in particular tropical forests or arid savannas. These environments had quite a bit to do with the general lack of fossils as conditions favored rapid decomposition due to deterioration and scavenging.
Proconsul has traits similar to modern monkeys and similar to modern apes. The genus proconsul existed from around 27mya-17mya. This genus is still quadrupedal, has forelimbs that are about equal to hindlimbs and a thin tooth enamel. The thin tooth enamel suggests a softer diet, either soft leaves or fruits. This species also shares some ape like traits like the lack of a tail, larger brain-to-body ratio, etcetera. Another trait in common is the ability to lift its hands above its head and the ability to turn its arms. Proconsul is also prognathic a trait that would exist in most other hominid species for some time. It is uncertain if this genus contained a direct hominid ancestor or if it was very close to a direct ancestor.
Following proconsul genus Afropithecus evolved in the Middle East and the first ape found outside of Africa is from Griphopithecus around 16.5mya and the first ape found in Europe is the Dryopithcus. While there is uncertainty of exact lineages in the previously mentioned genera there is almost certainly an ancestral line that includes Dryopithecus. The Y5 dental pattern is first found in this genus and is a primitive trait present in later hominids but not in later monkeys. This trait is also known as the Dryopithecus Pattern. This genera also had hands that would be good for grasping and suspensory locomotion as well as one of the first examples of a more vertical posture.
Sahelantropus, sometimes said to be the first biped lived around 7mya and it is debated whether or not it existed before or after the human chimpanzee split. Fossils for this genera are few. The chronologically next most likely hominid is Orrorin tugenensis, found in the Tugen Hills in Kenya, which has a slightly different fossil record. Orrorin lived between around 6.1 and 5.8mya. Some of the fossils suggest bipedalism and a diet that included meat. This species lived in a dry evergreen environment. If this was the first biped as some suggest it would mean that bipedalism appeared in an arboreal environment rather than a flat savanna. Robin Cromptom of the University of Liverpool believes this species moved ” by balancing and walking within the branches of the trees at times upright on two feet. ” Orrorin was also similar in size to modern chimpanzees. Debates about Orrorin pushed the split between hominins and apes to at least 7mya even though this is in disagreement with techniques involving the molecular clock. It seems there is much debate about this genus but on further reading it appears that it is a small group causing a disproportionate amount of the argument.
Ardipithecus occupies most of or all of the remaining gap between Orrorin and Australopithecus. Ardipithecus almost certainly arrived after the most recent common ancestor of humans and chimpanzees. This genus was around from around 5.8 to 4.4mya straddling the cusp of the Miocene and Pliocene. The occipital and temporal cranial remains indicate that the foramen magnum was most likely anteriorly located which lends itself towards bipedalism. If Ardipithecus wasn’t bipedal it certainly had traits that would later lend themselves to bipedal locomotion. Tooth enamel in Ardipithecus is thicker than modern chimpanzees as well though thinner than that of most other hominins. Scott Simpson says that Ardipithecus lived in “a mosaic of woodland and grasslands with lakes, swamps and springs nearby.” After Ardipithecus went away Australopithecus appeared.
Australopithecus is a genus of hominids that lived from around 4.4 to 2.15mya. First discovered in the Olduvai Gorge. Australopithecus is probably one of the more well know genus of early hominids and was probably the first hominid. Australopithecines were definitely habitually bipedal. They had a smaller brain, small canine teeth, thicker tooth enamel, sexual dimorphism, and stood up to 5′ tall. Australopith hands were capable of a wide degree of movement and had large thumbs compared to apes. Likely they were capable of precision gripping. The Laetoli footprints are often attributed to Australopithecus afarensis and indicate definite habitual bipedalism. One very famous fossil of Australopithecus is “Lucy” who lived around 3.2mya and was discovered in 1974. One of the important things about this discovery is that these fossils were able to show that bipedalism evolved before brain size. Australopithecus eventually gave way to Homo habilis.
Homo habilis the first species in the genus Homo lived from around 2.5 to 1.6mya while the Pleistocene epoch began. Compared to others in the Homo genus Homo habilis had long arms and was short. It’s brain was still small but it had become less prognathic. A very important development is often found in relation to H. habilis; stone tools. This species had a much more generalized diet than it’s ancestors. This species also moved out of Africa into Europe and Asia. H. habilis coexisted with H. erectus for some time as well.
Our ancestors started out as herbivores and as generalized apes so was their diets leading eventually to teeth and enamel suited for eating plant and meat matter and teeth that align well without requiring large diatomic gaps. The changes in teeth also likely effected social behaviors as our ancestors canines shrank. When one of us is mad we don’t tend to make a huge display showcasing our canines for example.
The genus Homo had a few branches that died off and only one that is still alive today. Us. All Homos were capable of hunting bigger game due to tool use but I don’t know of any evidence besides Homo sapiens. H. Neanderthal were making symbolic burials demonstrating both value for their departed and value for objects left with the dead. Composite tools allowed for more generalization of hunting and probably some plant gathering as well.
Social behavior was at least complex enough to justify or enable traveling long distances for rare materials that weren’t available in one’s home area in some cases to be traded off to another group for the desired materials. Copper, bronze, iron and eventually steel became part and parcel of human expansion and domination of the environment and one another. Instead of species of man dying off cultures died off, maybe were killed off.
Hominid and hominin species have tended to live in areas where fossilization wasn’t likely to occur though the geologic changes in some cases did work to preserve remains. Volcanoes, floods and desertification have all worked to preserve some of our ancestors remains as well as providing in many cases clear stratigraphy which aids greatly in dating these remains. In some cases where our ancestors were prey and not predator their remains would be drug off to a place conducive for fossilization. It is likely a trait common of our lineage to prefer environments where preservation of remains isn’t common.