The evolution of our species is a subject of great interest and controversy. Evolutionary history continues to be revised as more discoveries are made. If we know where we came from, perhaps we will have a better idea where we are going.
Humans are relative newcomers to planet Earth. Scientists believe that our planet is 4.5 billion years old, and bacterial life appeared and began to diversify 3.7 billion years ago. Hominid apes appeared 5 to 4 million years BCE (before the Christian Era). Remains of the first known hominid species, Australopithecus ramidus, were discovered in Ethiopia in 1995.
The famous “Lucy”, whose skeleton was found in 1974, is a member of the species Australopithicus afarensis, which lived in north east Africa from approximately 4 to 2.7 million years ago. She resembles a chimp. She is classified as ape, not human, but her skeletal structure indicates that she walked upright.
The Australopithicus africanus, which lived from before 3 million years ago to approximately 2 million years ago, has a larger brain and body than the Australopithicus afarensis, and is classified as an ape. Several well-preserved skulls exist, as well as many fragments. This species lived at the same time as early humans, and may have competed with them. Scholars disagree whether A. africanus is a direct ancestor of humans, or a related line which thrived for a time and disappeared about a million years ago. Primitive tools have been potassium-argon dated 3.1 and 2.5 million years ago, but there is no direct evidence that these erect bi-pedal apes used them.
Australopithicus robustus, a cluster of species which flourished 2.2 to 1 million years ago, evolved alongside early humans. Different variants continue to be discovered in Africa, some dating back as far as 4 million years. They were small-brained (about 400cc) vegetarians, and were prey for large predators, perhaps even our own ancestors, who enjoyed meat with their vegetables.
Homo habilis, who lived in east Africa from approximately 2.2 to 1.6 million years ago, is the considered the first human species. The few fossil remains show that their brains are about 30% larger than A. africanus brains. Males were much larger than females. They lived in open bush and savannah country, competing with large predators, and perhaps scavenging their prey. A skull found in 1972 is considered to be the oldest human individual ever discovered. It is believed that this species made and used tools. The “hunting hypothesis” speculates that evolution was boosted by the need of skill, strategy and co-ordination for a successful kill.
Homo erectus lived from approximately 2 million to around 400,000 years ago. Adult brains ranged from 900 to 1200 cc. Their bodies are modern looking, and much taller than their predecessors. “Lucy” was only 3’8″, but a homo erectus skeleton of a teen-age boy in east Africa measures 5’4″. Homo erectus was a tool-maker, and may have been the first species to use and control fire, which allowed migration into colder climates. Some time after one million years ago, this species moved out of Africa to Europe and Asia, where it was able of adapt to a variety of habitats.
The transition from Homo erectus to Homo sapiens occurred approximately 300,000 to 400,000 years ago. Fossil records show wooden tools and weapons, evidence of a hunting life-style. There is scholarly disagreement whether homo sapiens developed from the homo erectus populations in Africa, Europe and Asia, or evolved in Africa and migrated from there. DNA studies are underway to clarify this as well as the origin of racial differences.
Homo sapiens neandertalensis, named after the Neander valley where the first skull was found in 1857, is an anomaly in the story of human descent. Are Neanderthals a sub-species of modern humans, or a type of Homo erectus? One thing is clear they were tough and adaptable. They lived in Europe and western Asia from approximately 135,000 to 30,000 years ago, and survived through the Ice Age. They are well-known for their sloping foreheads. It is not clear why they died out.
Finally, homo sapiens sapiens appeared around 130,000 BCE, and continued into the present. They were less robust than the Neanderthals, but clever and adaptable enough to out-live them and other hominids. They made good use of tools and weapons, developed the use of jewelry and the beginnings of civilization, and left drawings on cave walls.
Can we trust scientific speculation of what happened millions and even billions of years ago? We can approach this question with skepticism, or with wonder. Clearly, we still have much to learn.
Sources and resources for further reading:
http://www.wsu.edu/gened/learn-modules/top_longfor/timeline/timeline.html
timeline
http://pubs.usgs.gov/gip/geotime/age.html
age of the earth
http://www.ucmp.berkeley.edu/exhibits/historyoflife.php
life on earth
