The timeline of human evolution outlines the major events in the evolutionary lineage of the modern human species, Homo sapiens, throughout the history of life, beginning some 4 billion years ago down to recent evolution within H. sapiens during and since the Last Glacial Period.
It includes brief explanations of the various taxonomic ranks in the human lineage. 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.
Overview of taxonomic ranks A tabular overview of the taxonomic ranking of Homo sapiens (with age estimates for each rank) is shown below.
Rank Name Common name StartedLife3.9 ByaArchaea3.8 ByaDomainEukaryotaEukaryotes2.1 ByaOpimodaExcludes Plants and their relatives1.54 ByaAmorpheaExcludes CRuMs1.45 ByaObazoaExcludes Amoebozoa1.3 ByaOpisthokontaHolozoa + Holomycota (Cristidicoidea and Fungi)1 ByaHolozoaExcludes Holomycota985 MyaFilozoaChoanozoa + Filasterea965 MyaChoanozoaChoanoflagellates + Animals950 MyaKingdomAnimaliaAnimals665 MyaSubkingdomEumetazoaExcludes Porifera (Sponges)ParahoxozoaExcludes Ctenophora (Comb Jellies)BilateriaTriploblasts / Worms567 MyaNephrozoa558 MyaDeuterostomiaDivision from Protostomes557 MyaPhylumChordataChordates (Vertebrates and closely related invertebrates)530 MyaOlfactoresExcludes cephalochordates (Lancelets)SubphylumVertebrataFish / Vertebrates505 MyaInfraphylumGnathostomataJawed fish460 MyaTeleostomiBony fish436 MyaSarcopterygiiLobe finned fish425 MyaSuperclassTetrapodaTetrapods (animals with four limbs)395 Mya AmniotaAmniotes (fully terrestrial tetrapods whose eggs are "equipped with an amnion") 340 MyaSynapsidaProto-Mammals318 MyaTherapsidaLimbs beneath the body and other mammalian traits280 MyaClassMammaliaMammals220 MyaSubclassTheriaMammals that give birth to live young (i.e. non-egg-laying) 160 MyaInfraclassEutheriaPlacental mammals (i.e. non-marsupials) 125 MyaMagnorderBoreoeutheriaSupraprimates, (most) hoofed mammals, (most) carnivorous mammals, cetaceans, and bats 115 MyaSuperorderEuarchontogliresSupraprimates: primates, colugos, tree shrews, rodents, and rabbits 100 MyaGrandorderEuarchontaPrimates, colugos, and tree shrews 90 MyaMirorderPrimatomorphaPrimates and colugos79.6 MyaOrderPrimatesPrimates / Plesiadapiformes66 MyaSuborderHaplorhini"Dry-nosed" (literally, "simple-nosed") primates: tarsiers and monkeys (incl. apes) 63 MyaInfraorderSimiiformesmonkeys (incl. apes) 40 MyaParvorderCatarrhini"Downward-nosed" primates: apes and old-world monkeys30 MyaSuperfamilyHominoideaApes: great apes and lesser apes (gibbons)21 MyaFamilyHominidaeGreat apes: humans, chimpanzees, gorillas and orangutans—the hominids17 MyaSubfamilyHomininaeHumans, chimpanzees, and gorillas (the African apes)12.5 MyaTribeHomininiIncludes both Homo and Pan (chimpanzees), but not Gorilla. 7 MyaSubtribeHomininaGenus Homo and close human relatives and ancestors after splitting from Pan—the hominins6.1 Mya(Genus)Ardipithecus s.l.5.77 Mya(Genus)Australopithecus4.3 MyaGenusHomo (H. habilis) 2.8 Mya(Species)H. erectus s.l.2 Mya(Species)H. heidelbergensis s.l.700,000 yaSpeciesHomo sapiens s.s. Humans300,000 yaDepending on the classification of the Homo heidelbergensis lineage; 0.8 if Neanderthals are classed as H. sapiens neanderthalensis, or if H. sapiens is defined cladistically from the divergence from H. neanderthalensis, 0.3 based on the available fossil evidence.
Timeline
Unicellular life Date Event 3.9 Bya The earliest life appears, possibly as protocells. Their genetic material was probably composed of RNA, capable of both self replication and enzymatic activity; their membranes were composed of lipids. The genes were separate strands, translated into proteins and often exchanged between the protocells. Prokaryotic cells appear; their genetic materials are composed of the more stable DNA and they use proteins for various reasons, primarily for aiding DNA to replicate itself by proteinaceous enzymes (RNA now acts as an intermediary in this central dogma of genetic information flow of cellular life); genes are now linked in sequences so all information passes to offsprings. They had cell walls & outer membranes and were probably initially thermophiles.3.8 ByaLast Universal Common Ancestor. All organisms alive today are descended from them. Its descendants split into Archaea and Bacteria.3.55 ByaAsgard archaea appear 2.2 ByaOrigin of the eukaryotes: organisms with nuclei, endomembrane systems (including mitochondria) and complex cytoskeletons; they spliced mRNA between transcription and translation (splicing also occurs in prokaryotes, but it is only of non-coding RNAs). The evolution of eukaryotes, and possibly sex, is thought to be related to the GOE, as it probably pressured two or three lineages of prokaryotes (including an aerobe one, which later became mitochondria) to depend on each other, leading to endosymbiosis. Early eukaryotes lost their cell walls and outer membranes. 1.6 ByaSexual reproduction evolves (mitosis and meiosis) around this time, leading to faster evolution where genes are mixed in every generation enabling greater variation for subsequent selection 1.3 ByaLast common ancestor with Amoebozoa 1 ByaLast common ancestor with Holomycota 950 MyaThe Choanozoa lineage of Filozoa evolves many features for making cell colonies. Last common ancestor with Choanoflagellates.Dawkins, R. (2005), The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution, Houghton Mifflin Harcourt, "Proterospongia is a rare freshwater protist, a colonial member of the Choanoflagellata." "Proterospongia itself is not the ancestor of sponges. However, it serves as a useful model for what the ancestor of sponges and other metazoans may have been like." https://www.ucmp.berkeley.edu/protista/proterospongia.html Berkeley University
Animals Date Event 665 MyaUrmetazoan: The first fossils that might represent animals appear in the 665 million year old rocks of the Trezona Formation of South Australia. These fossils are interpreted as being early sponges. Multicellular animals may have existed starting around 800 million years ago.655 MyaSeparation from the sponge lineage. Eumetazoa/Diploblast: separation from the Ctenophora ("comb jellies") lineage. 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. 567 MyaUrbilaterian: the last common ancestor of xenacoelomorphs, protostomes (including the arthropod [insect, crustacean, spider], mollusc [squid, snail, clam] and annelid [earthworm] lineages) and the deuterostomes (including vertebrates) (the last two are more related to each other and called Nephrozoa). Xenacoelomorphs all have a gonopore to expel gametes but nephrozoans merged it with their anus. 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. Reproductive tissue probably concentrates into a pair of gonads connecting just before the posterior orifice. "Cup-eyes" and balance organs evolve (the function of hearing added later as the more complex inner ear evolves in vertebrates). The nephrozoan through-gut had a wider portion in the front, called the pharynx. The integument or skin consists of an epithelial layer (epidermis) and a connective layer.558 MyaLast common ancestor with Protostomes 538.8 MyaMost known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, probably caused by long scale oxygenation since around 538.8 million years ago (sometimes called the Neoproterozoic Oxygenation Event or NOE) and also an influx of oceanic minerals. Deuterostomes, the last common ancestor of the Chordata [human] lineage, Hemichordata (acorn worms and graptolites) and Echinodermata (starfish, sea urchins, sea cucumbers, etc.), probably had both ventral and dorsal nerve cords like modern acorn worms.
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. Deuterostomes also evolved pharyngeal slits, which were probably used for filter feeding like in hemi- and proto-chordates.
Chordates Date Event 518 MyaThe increased amount of oxygen causes many eukaryotes, including most animals, to become obligate aerobes.
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."Obviously vertebrates must have had ancestors living in the Cambrian, but they were assumed to be invertebrate forerunners of the true vertebrates — proto-chordates. Pikaia has been heavily promoted as the oldest fossil protochordate." Richard Dawkins 2004 The Ancestor's Tale p. 289, Other, earlier chordate predecessors include Myllokunmingia fengjiaoa, Yunnanozoon lividum, and Haikouichthys ercaicunensis. 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. All chordates except tunicates sport an intricate, closed circulatory system, with highly branched blood vessels.