Timeline of human evolution

Planulozoa/ParaHoxozoa: separation from the Placozoa and Cnidaria lineages. All diploblasts possess epithelia, nerves, muscles, and connective tissue and mouths, and except for placozoans, have some form of symmetry, with their ancestors probably having radial symmetry like that of cnidarians. Diploblasts separated their early embryonic cells into two germ layers (ecto- and endoderm). Photoreceptive eye-spots evolve.

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. Acorn worms have a plexus concentrated into both dorsal and ventral nerve cords. The dorsal cord reaches into the proboscis, and is partially separated from the epidermis in that region. This part of the dorsal nerve cord is often hollow, and may well be homologous with the brain of vertebrates.^[9] Deuterostomes also evolved pharyngeal slits, which were probably used for filter feeding like in hemi- and proto-chordates.

The Chordata ancestor gave rise to the lancelets (Amphioxii) and Olfactores. Ancestral chordates evolved a post-anal tail, notochord, and endostyle (precursor of thyroid). The pharyngeal slits (or gills) are now supported by connective tissue and used for filter feeding and possibly breathing. The first of these basal chordates to be discovered by science was Pikaia gracilens.^[10] Other, earlier chordate predecessors include Myllokunmingia fengjiaoa,^[11] Yunnanozoon lividum,^[12] and Haikouichthys ercaicunensis.^[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.^[14] All chordates except tunicates sport an intricate, closed circulatory system, with highly branched blood vessels.

Placoderms had competition with the previous dominant animals, the cephalopods and sea scorpions, and rose to dominance themselves. A lineage of them probably evolved into the bony and cartilaginous fish, after evolving scales, teeth (which allowed the transition to full carnivory), stomachs, spleens, thymuses, myelin sheaths, hemoglobin and advanced, adaptive immunity (the latter two occurred independently in the lampreys and hagfish). Jawed fish also have a third, lateral semicircular canal and their otoliths are divided between a saccule and utricle.

Some freshwater lobe-finned fish (sarcopterygii) develop limbs and give rise to the Tetrapodomorpha. These fish evolved in shallow and swampy freshwater habitats, where they evolved large eyes and spiracles.

Primitive tetrapods ("fishapods") developed from tetrapodomorphs with a two-lobed brain in a flattened skull, a wide mouth and a medium snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed adaptations of fins with fleshy bases and bones. (The "living fossil" coelacanth is a related lobe-finned fish without these shallow-water adaptations.) Tetrapod fishes used their fins as paddles in shallow-water habitats choked with plants and detritus. The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water.^[18]

Panderichthys is a 90–130 cm (35–50 in) long fish from the Late Devonian period (380 Mya). It has a large tetrapod-like head. Panderichthys exhibits features transitional between lobe-finned fishes and early tetrapods.

Trackway impressions made by something that resembles Ichthyostega's limbs were formed 390 Ma in Polish marine tidal sediments. This suggests tetrapod evolution is older than the dated fossils of Panderichthys through to Ichthyostega.

Tiktaalik is a genus of sarcopterygian (lobe-finned) fishes from the late Devonian with many tetrapod-like features. It shows a clear link between Panderichthys and Acanthostega.

Acanthostega is an extinct tetrapod, among the first animals to have recognizable limbs. It is a candidate for being one of the first vertebrates to be capable of coming onto land. It lacked wrists, and was generally poorly adapted for life on land. The limbs could not support the animal's weight. Acanthostega had both lungs and gills, also indicating it was a link between lobe-finned fish and terrestrial vertebrates. The dorsal pair of ribs form a rib cage to support the lungs, while the ventral pair disappears.

Ichthyostega is another extinct tetrapod. Being one of the first animals with only two pairs of limbs (also unique since they end in digits and have bones), Ichthyostega is seen as an intermediate between a fish and an amphibian. Ichthyostega had limbs but these probably were not used for walking. They may have spent very brief periods out of water and would have used their limbs to paw their way through the mud.^[19] They both had more than five digits (eight or seven) at the end of each of their limbs, and their bodies were scaleless (except their bellies, where they remained as gastralia). Many evolutionary changes occurred at this stage: eyelids and tear glands evolved to keep the eyes wet out of water and the eyes became connected to the pharynx for draining the liquid; the hyomandibula (now called columella) shrank into the spiracle, which now also connected to the inner ear at one side and the pharynx at another, becoming the Eustachian tube (columella assisted in hearing); an early eardrum (a patch of connective tissue) evolved on the end of each tube (called the otic notch); and the ceratohyal and basihyal merged into the hyoid. These "fishapods" had more ossified and stronger bones to support themselves on land (especially skull and limb bones). Jaw bones fuse together while gill and opercular bones disappear.

Pederpes from around 350 Ma indicates that the standard number of 5 digits evolved at the Early Carboniferous, when modern tetrapods (or "amphibians") split in two directions (one leading to the extant amphibians and the other to amniotes). At this stage, our ancestors evolved vomeronasal organs, salivary glands, tongues, parathyroid glands, three-chambered hearts (with two atria and one ventricle) and bladders, and completely removed their gills by adulthood. The glottis evolves to prevent food going into the respiratory tract. Lungs and thin, moist skin allowed them to breathe; water was also needed to give birth to shell-less eggs and for early development. Dorsal, anal and tail fins all disappeared.

Lissamphibia (extant amphibians) retain many features of early amphibians but they have only four digits (caecilians have none).

The Late Carboniferous saw the emergence of Amniota, The split between the two major groups of amniotes, the ancestors of reptiles and birds (Sauropsida) and mammals (Synapsida) likely occurred by around 310 million years ago.^[20] The skin of the common ancestor of amniotes was covered in reptile-like scales.^[21]

Evolution of the amniotic egg allows the amniotes to reproduce on land and lay shelled eggs on dry land. They did not need to return to water for reproduction nor breathing. 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. At this stage, adrenal tissue may have concentrated into discrete glands.

Amniotes have advanced nervous systems, with twelve pairs of cranial nerves, unlike lower vertebrates. They also evolved true sternums but lost their eardrums and otic notches (hearing only by columella bone conduction).

Monotremes are an egg-laying group of mammals represented today by the platypus and echidna. 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. Comparing this to other mammals, it can be inferred that the first mammals to gain sexual differentiation through the existence or lack of SRY gene (found in the y-Chromosome) evolved only in the therians. 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. Nipples stemmed out of the therian milk lines. The posterior orifice separates into anal and urogenital openings; males possess an external penis.

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.^[27]

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. They reduced the number of mammaries to only two pairs (on the chest). Primatomorpha is a subdivision of Euarchonta including primates and their ancestral stem-primates Plesiadapiformes. 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.

The Plesiadapiformes very likely contain the ancestor species of all primates.^[28] 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.^[29][30]

One of the last Plesiadapiformes is Carpolestes simpsoni, having grasping digits but not forward-facing eyes.

Simians split into infraorders Platyrrhini and Catarrhini. They fully transitioned to diurnality and lacked any claw and tapetum lucidum (which evolved many times in various vertebrates). They possibly evolved at least some of the paranasal sinuses, and transitioned from estrous cycle to menstrual cycle. The number of mammaries is now reduced to only one thoracic pair. Platyrrhines, New World monkeys, have prehensile tails and males are color blind. 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^[31]). Catarrhines mostly stayed in Africa as the two continents drifted apart. Possible early ancestors of catarrhines include Aegyptopithecus and Saadanius.

Catarrhini splits into 2 superfamilies, Old World monkeys (Cercopithecoidea) and apes (Hominoidea). Human trichromatic color vision had its genetic origins in this period. Catarrhines lost the vomeronasal organ (or possibly reduced it to vestigial status).

Proconsul was an early genus of catarrhine primates. They had a mixture of Old World monkey and ape characteristics. Proconsul's monkey-like features include thin tooth enamel, a light build with a narrow chest and short forelimbs, and an arboreal quadrupedal lifestyle. Its ape-like features are its lack of a tail, ape-like elbows, and a slightly larger brain relative to body size.

Proconsul africanus is a possible ancestor of both great and lesser apes, including humans.

Pierolapithecus catalaunicus is thought to be a common ancestor of humans and the other great apes, or at least a species that brings us closer to a common ancestor than any previous fossil discovery. 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.

Hominini: The latest common ancestor of humans and chimpanzees is estimated to have lived between roughly 10 to 5 million years ago. Both chimpanzees and humans have a larynx that repositions during the first two years of life to a spot between the pharynx and the lungs, indicating that the common ancestors have this feature, a precondition for vocalized speech in humans. Speciation may have begun shortly after 10 Ma, but late admixture between the lineages may have taken place until after 5 Ma. Candidates of Hominina or Homininae species which lived in this time period include Graecopithecus (c. 7 Ma), Sahelanthropus tchadensis (c. 7 Ma), Orrorin tugenensis (c. 6 Ma).

Ardipithecus was arboreal, meaning it lived largely in the forest where it competed with other forest animals for food, no doubt including the contemporary ancestor of the chimpanzees. 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. Australopithecus afarensis lived between 3.9 and 2.9 million years ago, and is considered one of the earliest hominins—those species that developed and comprised the lineage of Homo and Homo's closest relatives after the split from the line of the chimpanzees.

It is thought that A. afarensis was ancestral to both the genus Australopithecus and the genus Homo. Compared to the modern and extinct great apes, A. afarensis had reduced canines and molars, although they were still relatively larger than in modern humans. A. afarensis also has a relatively small brain size (380–430 cm³) and a prognathic (anterior-projecting) face.

Australopithecines have been found in savannah environments; they probably developed their diet to include scavenged meat. Analyses of Australopithecus africanus lower vertebrae suggests that these bones changed in females to support bipedalism even during pregnancy.

Early Homo appears in East Africa, speciating from australopithecine ancestors. The Lower Paleolithic is defined by the beginning of use of stone tools. Australopithecus garhi was using stone tools at about 2.5 Ma. Homo habilis is the oldest species given the designation Homo, by Leakey et al. in 1964. H. habilis is intermediate between Australopithecus afarensis and H. erectus, and there have been suggestions to re-classify it within genus Australopithecus, as Australopithecus habilis.

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. It is currently unassigned to a species, and it is unclear if it represents the ancestor to H. habilis and H. rudolfensis, which are estimated to have evolved around 2.4 Ma.^[40]

Stone tools found at the Shangchen site in China and dated to 2.12 million years ago are considered the earliest known evidence of hominins outside Africa, surpassing Dmanisi hominins found in Georgia by 300,000 years, although whether these hominins were an early species in the genus Homo or another hominin species is unknown.^[41]

Homo erectus derives from early Homo or late Australopithecus.

Homo habilis, although significantly different of anatomy and physiology, is thought to be the ancestor of Homo ergaster, or African Homo erectus; but it is also known to have coexisted with H. erectus for almost half a million years (until about 1.5 Ma). From its earliest appearance at about 1.9 Ma, H. erectus is distributed in East Africa and Southwest Asia (Homo georgicus). H. erectus is the first known species to develop control of fire, by about 1.5 Ma.

H. erectus later migrates throughout Eurasia, reaching Southeast Asia by 0.7 Ma. It is described in a number of subspecies.^[42] Early humans were social and initially scavenged, before becoming active hunters. The need to communicate and hunt prey efficiently in a new, fluctuating environment (where the locations of resources need to be memorized and told) may have driven the expansion of the brain from 2 to 0.8 Ma.

Evolution of dark skin at about 1.2 Ma.^[43]

Homo antecessor may be a common ancestor of Homo sapiens and Neanderthals.^[44][45] At present estimate, humans have approximately 20,000–25,000 genes and share 99% of their DNA with the now extinct Neanderthal^[46] and 95–99% of their DNA with their closest living evolutionary relative, the chimpanzees.^[47][48] The human variant of the FOXP2 gene (linked to the control of speech) has been found to be identical in Neanderthals.^[49]

Divergence of Neanderthal and Denisovan lineages from a common ancestor.^[50] Homo heidelbergensis (in Africa also known as Homo rhodesiensis) had long been thought to be a likely candidate for the last common ancestor of the Neanderthal and modern human lineages. However, genetic evidence from the Sima de los Huesos fossils published in 2016 seems to suggest that H. heidelbergensis in its entirety should be included in the Neanderthal lineage, as "pre-Neanderthal" or "early Neanderthal", while the divergence time between the Neanderthal and modern lineages has been pushed back to before the emergence of H. heidelbergensis, to about 600,000 to 800,000 years ago, the approximate age of Homo antecessor.^[51][52] Brain expansion (enlargement) between 0.8 and 0.2 Ma may have occurred due to the extinction of most African megafauna (which made humans feed from smaller prey and plants, which required greater intelligence due to greater speed of the former and uncertainty about whether the latter were poisonous or not), extreme climate variability after Mid-Pleistocene Transition (which intensified the situation, and resulted in frequent migrations), and in general selection for more social life (and intelligence) for greater chance of survival, reproductivity, and care for mothers. Solidified footprints dated to about 350 ka and associated with H. heidelbergensis were found in southern Italy in 2003.^[53]

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). By 200 ka, humans had stopped their brain expansion.

Neanderthals and Denisovans emerge from the northern Homo heidelbergensis lineage around 500-450 ka while sapients emerge from the southern lineage around 350-300 ka.^[54]

Fossils attributed to H. sapiens, along with stone tools, dated to approximately 300,000 years ago, found at Jebel Irhoud, Morocco^[55] yield the earliest fossil evidence for anatomically modern Homo sapiens. Modern human presence in East Africa (Gademotta), at 276 kya.^[56] In July 2019, anthropologists reported the discovery of 210,000 year old remains of what may possibly have been a H. sapiens in Apidima Cave, Peloponnese, Greece.^[57][58][59]

Patrilineal and matrilineal most recent common ancestors (MRCAs) of living humans roughly between 200 and 100 kya^[60][61] with some estimates on the patrilineal MRCA somewhat higher, ranging up to 250 to 500 kya.^[62]

160,000 years ago, Homo sapiens idaltu in the Awash River Valley (near present-day Herto village, Ethiopia) practiced excarnation.^[63]

Modern human presence in Southern Africa and West Africa.^[64] Appearance of mitochondrial haplogroup (mt-haplogroup) L2.

Early evidence for behavioral modernity.^[65] Appearance of mt-haplogroups M and N. Southern Dispersal migration out of Africa, Proto-Australoid peopling of Oceania.^[66] Archaic admixture from Neanderthals in Eurasia,^[67][68] from Denisovans in Oceania with trace amounts in Eastern Eurasia,^[69] and from an unspecified African lineage of archaic humans in Sub-Saharan Africa as well as an interbred species of Neanderthals and Denisovans in Asia and Oceania.^{[70][71][72][73]}

Behavioral modernity develops by this time or earlier, according to the "great leap forward" theory.^[74] Extinction of Homo floresiensis.^[75] M168 mutation (carried by all non-African males). Appearance of mt-haplogroups U and K. Peopling of Europe, peopling of the North Asian Mammoth steppe. Paleolithic art. Extinction of Neanderthals and other archaic human variants (with possible survival of hybrid populations in Asia and Africa). Appearance of Y-Haplogroup R2; mt-haplogroups J and X.

Last Glacial Maximum; Epipaleolithic / Mesolithic / Holocene. Peopling of the Americas. Appearance of: Y-Haplogroup R1a; mt-haplogroups V and T. Various recent divergence associated with environmental pressures, e.g. light skin in Europeans and East Asians (KITLG, ASIP), after 30 ka;^[76] Inuit adaptation to high-fat diet and cold climate, 20 ka.^[77]

Extinction of late surviving archaic humans at the beginning of the Holocene (12 ka). 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,^[78] or lactase persistence.^[79][80] The past 100,000 years have seen selective reductions in brain size for some human lineages during warmer interglacial periods.^[81]