Timeline of human evolution

The timeline reflects the mainstream views in modern taxonomy, based on the principle of phylogenetic nomenclature; in cases of open questions with no clear consensus, the main competing possibilities are briefly outlined.

The Holozoa lineage of eukaryotes evolves many features for making cell colonies, and finally leads to the ancestor of animals (metazoans) and choanoflagellates.

Earliest development of bilateral symmetry, mesoderm, head (anterior cephalization) and various gut muscles (and thus peristalsis) and, in the Nephrozoa, nephridia (kidney precursors), coelom (or maybe pseudocoelom), distinct mouth and anus (evolution of through-gut), and possibly even nerve cords and blood vessels.

An archaic survivor from this stage is the acorn worm, sporting an open circulatory system (with less branched blood vessels) with a heart that also functions as a kidney.

[13] They probably lost their ventral nerve cord and evolved a special region of the dorsal one, called the brain, with glia becoming permanently associated with neurons.

They probably evolved the first blood cells (probably early leukocytes, indicating advanced innate immunity), which they made around the pharynx and gut.

[15] They depended on gills for respiration and evolved the unique sense of taste (the remaining sense of the skin now called "touch"), endothelia, camera eyes and inner ears (capable of hearing and balancing; each consists of a lagena, an otolithic organ and two semicircular canals) as well as livers, thyroids, kidneys and two-chambered hearts (one atrium and one ventricle).

The gap between the first and second arches just below the braincase (fused with upper jaw) created a pair of spiracles, which opened in the skin and led to the pharynx (water passed through them and left through gills).

Lungs and thin, moist skin allowed them to breathe; water was also needed to give birth to shell-less eggs and for early development.

This adaptation and the desiccation-resistant scales gave them the capability to inhabit the uplands for the first time, albeit making them drink water through their mouths.

The therapsids had temporal fenestrae larger and more mammal-like than pelycosaurs, their teeth showed more serial differentiation, their gait was semi-erect and later forms had evolved a secondary palate.

Most early mammals were small shrew-like animals that fed on insects and had transitioned to nocturnality to avoid competition with the dominant archosaurs — this led to the loss of the vision of red and ultraviolet light (ancestral tetrachromacy of vertebrates reduced to dichromacy).

Teeth fully differentiate into incisors, canines, premolars and molars; mammals become diphyodont and possess developed diaphragms and males have internal penises.

Recent genome sequencing of the platypus indicates that its sex genes are closer to those of birds than to those of the therian (live birthing) mammals.

Early mammals and possibly their eucynodontian ancestors had epipubic bones, which serve to hold the pouch in modern marsupials (in both sexes).

Evolution of live birth (viviparity), with early therians probably having pouches for keeping their undeveloped young like in modern marsupials.

Monotremes and therians independently detach the malleus and incus from the dentary (lower jaw) and combine them to the shrunken columella (now called stapes) in the tympanic cavity behind the eardrum (which is connected to the malleus and held by another bone detached from the dentary, the tympanic plus ectotympanic), and coil their lagena (cochlea) to advance their hearing, with therians further evolving an external pinna and erect forelimbs.

Female placentalian mammals do not have pouches and epipubic bones but instead have a developed placenta which penetrates the uterus walls (unlike marsupials), allowing a longer gestation; they also have separated urinary and genital openings.

[23] A group of small, nocturnal, arboreal, insect-eating mammals called Euarchonta begins a speciation that will lead to the orders of primates, treeshrews and flying lemurs.

An early stem-primate, Plesiadapis, still had claws and eyes on the side of the head, making it faster on the ground than in the trees, but it began to spend long times on lower branches, feeding on fruits and leaves.

[24] They first appeared in the fossil record around 66 million years ago, soon after the Cretaceous–Paleogene extinction event that eliminated about three-quarters of plant and animal species on Earth, including most dinosaurs.

The individuals whose descendants would become Platyrrhini are conjectured to have migrated to South America either on a raft of vegetation or via a land bridge (the hypothesis now favored[27]).

Proconsul's monkey-like features include thin tooth enamel, a light build with a narrow chest and short forelimbs, and an arboreal quadrupedal lifestyle.

It had the special adaptations for tree climbing as do present-day humans and other great apes: a wide, flat rib cage, a stiff lower spine, flexible wrists, and shoulder blades that lie along its back.

Ardipithecus was probably bipedal as evidenced by its bowl shaped pelvis, the angle of its foramen magnum and its thinner wrist bones, though its feet were still adapted for grasping rather than walking for long distances.

A member of the Australopithecus afarensis left human-like footprints on volcanic ash in Laetoli, northern Tanzania, providing strong evidence of full-time bipedalism.

LD 350-1 is now considered the earliest known specimen of the genus Homo, dating to 2.75–2.8 Ma, found in the Ledi-Geraru site in the Afar Region of Ethiopia.

[49] H. sapiens lost the brow ridges from their hominid ancestors as well as the snout completely, though their noses evolve to be protruding (possibly from the time of H. erectus).

[58] 160,000 years ago, Homo sapiens idaltu in the Awash River Valley (near present-day Herto village, Ethiopia) practiced excarnation.

Accelerated divergence due to selection pressures in populations participating in the Neolithic Revolution after 12 ka, e.g. East Asian types of ADH1B associated with rice domestication,[74] or lactase persistence.