Human Evolution: Timeline, Stages, Theories & Evidence

The theory of evolution forms the basis of virtually every other idea in contemporary biology, from the startlingly close similarities between dinosaurs and birds to mechanism of antibiotic resistance. Charles Darwin's name is essentially synonymous with the concept, but it was in fact the combined brainchild of Darwin and the much less heralded Alfred Russell Wallace, who independently arrived at the notion of natural selection.

Wallace and Darwin wound up collaborating on an 1858 publication that preceded Darwin's magnum opus, On the Origin of Species.

The idea of evolution was controversial in its day and remains so today, mainly because it encompasses humans as well as all of the other forms of life on Earth, in some ways dispensing of the notion that humans enjoy an exalted place in the pantheon of living things.

Nevertheless, the evidence for human evolution, and the fact that humans evolved from a primate common ancestor, is as scientifically unassailable as anything else in biology, physics, chemistry or any other field of scientific inquiry.

Above all, learning the facts about human origins is fascinating beyond measure.

Evolution, in the world of biology, refers to "descent with modification," a process that is reliant on natural selection. Natural selection in turn refers to the ability of organisms that possess favorable traits within their own environment to survive better than other animals in the same environment. This includes other animals in the same species that do not possess these traits. Evolution can be defined as a change in the frequency of genes in a population over time.

A typical example is a group of giraffes that eat from the leafy branches of trees.

Those that happen to possess longer necks will be able to feed themselves more readily, leading to a higher survival rate among these giraffes. Because giraffe neck length is a heritable trait, meaning that it can be passed to the next generation through genes encoded in deoxyribonucleic acid (DNA, the "genetic material" in all living things on the planet), longer-necked giraffes become more prevalent in this group, and those with shorter necks accordingly die off.

Importantly, natural selection is not a process of conscious striving; it is a matter of luck, with nature choosing the organisms that are the "fittest" in reproductive terms. In addition, an animal that may be "strong" in one setting may find conditions in another immediately lethal. Humans and virtually all other organisms, for example, would be unable to survive in the deep-water thermal vents in which certain bacteria-like organisms can live.

All organisms are descended from a common ancestor, and humans, being primates, share a common ancestor with other primates that lived relatively recently in the grand scheme of life. The first living things appeared on earth some 3.5 billion years ago, "only" a billion or so years after the Earth itself formed. Modern humans share a common ancestor with other apes of today that lived about 6 million to 8 million years ago.

Most of the evidence for evolution of human beings comes from fossil evidence, and this evidence has been strongly reinforced by the methods of modern molecular biology, such as DNA analysis. The structure of DNA was not confirmed until the 1950s, about 100 years after Darwin and Wallace first arrived at the mechanism by which evolution occurs at the cellular level.

Paleoanthropology is the scientific study of human evolution that combines paleontology (the examination and analysis of the fossil record) with the study of human cultures and societies through the lens of biology (anthropology). Paleoanthropologists, then, are scientists who analyze early species of hominids, or early humans.

Some 15 to 20 known hominid species arose over a considerable period of time before modern humans evolved into the 7 billion or so people that populate the planet as of the close of the 21st century's second decade. All but one of these, despite their considerable ingenuity and resourcefulness in comparison to their forebears and non-hominid contemporaries, went extinct.

Importantly, apes are not distinct from humans; instead, humans are a kind of ape, just as humans are a type of primate, mammal and so on up the taxonomic classification chain.

But for explanatory purposes here, humans and apes will be treated as distinct life forms. Other apes include chimpanzees, bonobos ("pygmy chimps"), gorillas, orangutans and gibbons.

The first four of these are known as the "great apes" because of their larger size.

As hominids evolved over time, the world witnessed the emergence of primates that combine apelike and human features, with the gradual loss of more apelike features for humanlike features.

Common features of apes are a strong brow, an elongated skull, incomplete bipedalism (i.e., "knuckle-walking"), smaller brains, larger canine teeth and a sloping face. Common human features, in contrast, are a shorter face, a non-elongated skull, larger brains, a more complex cultural and community system, small canine teeth, a spinal cord positioned more directly underneath the skull (a trait indicative of bipedalism) and the use of stone tools.

The first primates appeared some 55 million years ago, about 10 million years after the last dinosaurs walked the Earth. The first orangutans split from what became the human branch of the primate family tree perhaps 10 million years ago; gorillas arrived on the scene about 8 million years ago and split from humans' common ancestor.

Among the apes, the closest relatives of humans are bonobos and chimpanzees, as established by both the fossil record and DNA evidence. The common ancestor of humans, chimpanzees and bonobos that evolved into being 6 million to 8 million years ago gave rise to a succession of ancestors of hominids (and hence of modern humans, or Homo sapiens) known as hominins.

The oldest apelike relative of humans originated in central Africa and dispersed around the globe from there.

• The 13-million-year-old skull of an infant primate, believed to be a common ancestor of all apes and humans, was found in Kenya in 2014.

Bipedalism, which is the ability to walk upright and one of the defining characteristics of hominids, first arose about 6 million years ago, but only became consistent and then obligatory some 4 million years ago.

Hominids first began forming their own tools about 2.6 million years ago, made purposeful use of fire starting about 800,000 years ago and experienced an accelerated increase in brain size between roughly 800,000 and 200,000 years ago.

Most modern human characteristics have evolved in the last 200,000 years, with a shift to farming and agricultural methods from hunting and gathering beginning about 12,000 years ago. This allowed humans to settle in one place and build elaborate social communities and well as reproduce and survive at a faster rate.

Fossils have provided paleoanthropologists with a wealth of knowledge about the hominin species and hominid predecessors of modern humans. Some have been placed in the genus Homo, while others belong to now-extinct genera. From oldest to most recent, some of the humanlike species that have graced Earth include:

**Sahelenthropus tchadensis.** All that exists now of this ancient creature that lived 6 to 7 million years ago are skull parts found in 2001 in west-central Africa. S. tchadensis had a chimp-sized brain, was able to walk on two legs (but was not completely bipedal), had a spinal opening underneath its cranium, featured smaller canine teeth and boasted a prominent brow ridge. It was thus very apelike.

Orrorin tugenensis. A skeleton of this hominin from 6.2 to 5.8 million years ago was also found in 2001, this one in eastern Africa. It had teeth and hands, was able to walk upright but was also arboreal (i.e., it climbed trees), had small human-like teeth and was size of a modern chimpanzee.

Ardipithecus kadabba. This human ancestor lived from 5.8 to 5.2 million years ago, with its remains (a jaw, teeth, hand and foot bones, and arm and clavicle bones) found in 1997 in eastern Africa. These remains establish that the new species was bipedal, and it lived in woodlands and grasslands, mostly the former (an apelike trait).

Ardipithecus ramidus. This creature lived about 4.4 million years ago, with some remains found in 1994 and a partial skeleton, named "Ardi," found in 2009. It walked upright but had opposable toes to climb trees, and lived in the woods.

Australopithecus afarensis. Known colloquially as "Lucy," A. afarensis was an eastern African dweller between 3.85 and 2.95 million years ago, making Lucy the longest-living pre-human species.

Over 300 individual A. afarensis pre-human fossils have been found, and they show that this hominin had rapid child growth and reached maturity faster than modern humans. Lucy had an apelike face, a larger brain than a chimp's but smaller than a modern human's, and small canines.

It was bipedal but could still climb trees; that it could live both in trees and on the ground allowed it to survive many extended climate changes. Lucy is thought to be among first early humans to live on the savannah, or grassy plain.

Australopithecus africanus. This hominin lived from 3.3 to 2.1 million years ago in southern Africa and was discovered in 1924. It had small, humanlike teeth, a larger brain and a rounder brain case (like humans have). However, this bipedal creature also had apelike features (e.g., long arms, a strong jutting jaw beneath a sloping face and shoulders and hands adapted for climbing).

Homo habilis. One of the earliest known ancestors in our own genus (Homo) and thus a hominid, "handy man" (the translation of the name from Latin) existed from 2.4 million to 1.4 million years ago in Eastern and Southern Africa. H. habilis is thought to be one of the first species to create stone tools; it had apelike features like long arms and an apelike face, but it also possessed a large brain case and small teeth, and it is known to have used tools.

**Homo erectus**. This species spread all over Africa and (out of Africa) into Asia 1.89 million to 143,000 years ago. The oldest species are often referred to as Homo ergaster. It had humanlike body proportions, ate a significant amount of meat as well as plants, lived almost solely on the ground and developed a progressively larger brain and brain case.

Fossil evidence showed that this early human cared for its young, old and sick, and was the longest-lived of all of the early hominid species. Its ability to walk and run long distances allowed it to spread far and wide.

**Homo heidelbergensis**. The first hominids in Europe, these hominids also lived in China and east Africa some 700,000 to 200,000 years ago; it was was the first species to live in colder climates, with short, wide bodies to retain heat.

These european hominids used tools and fire, built "homes" out of wood and rocks, was first species to hunt large animals, and was the direct ancestors of Neanderthals. H. heidelbergensis had a brain size comparable to that of modern humans.

Homo neanderthalensis. This is the famed Neanderthal and lived from about 400,000 to 40,000 years ago throughout Europe and parts of Asia. The closest extinct relative to Homo sapiens, it was shorter, more muscular and stockier than modern humans, and large noses to help with cold air. Neanderthals had a humanlike face, brains as large (or larger) than H. sapiens and lived in shelters such as caves.

It used tools and weapons, made and wore clothes, made "art" and buried its dead; evidence exists that the Neanderthals had primitive language and used symbols, establishing the earliest traces of what is now called culture.

Homo sapiens. Modern humans evolved in Africa spread worldwide 200,000 years ago, and have continued to evolve larger brains and lighter bodies over their evolutionary history. Human faces have also changed over time to have less pronounced jaws and brow lines, smaller teeth and smaller jaws. You are a member of this species.

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