Evolution of Homo habilis

One of several possible lines of descent, or taxonomic ranking, of *Homo sapiens* is shown below.

## Unicellular life

 0.9 Ga
The choanoflagellates may look similar to the ancestors of the entire animal kingdom, and in particular they may be the direct ancestors of sponges.
Proterospongia (members of the Choanoflagellata) are the best living examples of what the ancestor of all animals may have looked like. They live in colonies, and show a primitive level of cellular specialization for different tasks.

## Animalia

Urmetazoan:
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.
Separation from the Porifera (sponges) lineage.
Eumetazoa/Diploblast: separation from the Ctenophora ("comb jellies") lineage.
Planulozoa/ParaHoxozoa: separation from the Placozoa and Cnidaria lineages.
 570–550 Ma
Urbilaterian:
Bilateria/Triploblasts, Nephrozoa (555 Ma), last common ancestor of protostomes (including the arthropod [insect, crustacean] and platyzoan [flatworms] lineages) and the deuterostomes (including the vertebrate [human] lineage).
Earliest development of the brain, and of bilateral symmetry.
 541 Ma
Most known animal phyla appeared in the fossil record as marine species during the **Cambrian explosion**.
Deuterostomes, last common ancestor of the chordate [human] lineage, the Echinodermata (starfish, sea urchins, sea cucumbers, etc.) and Hemichordata (acorn worms and graptolites).
An archaic survivor from this stage is the acorn worm, sporting a circulatory system with a heart that also functions as a kidney. Acorn worms have a gill-like structure used for breathing, a structure similar to that of primitive fish. 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.

## Chordates

The lancelet, still living today, retains some characteristics of the primitive chordates. It resembles *Pikaia*.

 505 Ma
 480 Ma
 410 Ma

## Tetrapods

Further information: Evolution of tetrapods

Some fresh water lobe-finned fish (Sarcopterygii) develop legs and give rise to the Tetrapoda.
The first tetrapods evolved in shallow and swampy freshwater habitats.
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed brain in a flattened skull, a wide mouth and a short 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.

*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*.
 375 Ma
 365 Ma

*Acanthostega* is an extinct amphibian, 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.
*Ichthyostega* is an early tetrapod. Being one of the first animals with legs, arms, and finger bones, *Ichthyostega* is seen as a hybrid between a fish and an amphibian. *Ichthyostega* had legs but its limbs probably were not used for walking. They may have spent very brief periods out of water and would have used their legs to paw their way through the mud.
Amphibia were the first four-legged animals to develop lungs which may have evolved from *Hynerpeton* 360 Mya.
 300 Ma
From amphibians came the first reptiles: *Hylonomus* is the earliest known reptile. It was 20 cm (8 in) long (including the tail) and probably would have looked rather similar to modern lizards. It had small sharp teeth and probably ate millipedes and early insects. It is a precursor of later Amniotes and mammal-like reptiles. ?lpha keratin first evolves here. It is used in the claws of modern lizards and birds, and hair in mammals.
Evolution of the amniotic egg gives rise to the Amniota, reptiles that can reproduce on land and lay eggs on dry land. They did not need to return to water for reproduction. This adaptation gave them the capability to colonize the uplands for the first time.
Reptiles have advanced nervous systems, compared to amphibians, with twelve pairs of cranial nerves.

## Mammals

Further information: Evolution of mammals

The earliest **mammal-like reptiles** are the pelycosaurs. The pelycosaurs were the first animals to have temporal fenestrae. Pelycosaurs are not therapsids but soon they gave rise to them. The Therapsida were the direct ancestor of mammals.
 220 Ma
One sub-group of therapsids, the cynodonts, evolved more mammal-like characteristics.
 220 Ma
From Eucynodontia (cynodonts) came the first mammals. Most early mammals were small shrew-like animals that fed on insects. Although there is no evidence in the fossil record, it is likely that these animals had a constant body temperature and milk glands for their young. The neocortex region of the brain first evolved in mammals and thus is unique to them.
 160 Ma
 100 Ma

## Primates

Further information: Evolution of primates

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. 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. 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.
 63 Ma
 30 Ma
 25 Ma
Catarrhini splits into 2 superfamilies, Old World monkeys (Cercopithecoidea) and apes (Hominoidea). Our trichromatic color vision had its genetic origins in this period.
*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.

## Hominidae

 13 Ma
 10 Ma
 10–4 Ma
The latest common ancestor of humans and chimpanzees
is estimated to have lived between c. 10 and 4 Ma. 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
*Ouranopithecus* (c. 8 Ma),
*Graecopithecus* (c. 7 Ma),
*Sahelanthropus tchadensis* (c. 7 Ma),
*Orrorin tugenensis* (c. 6 Ma).

    *Ardipithecus* is, or may be, a very early hominin genus (tribe Hominini and subtribe Hominina). Two species are described in the literature: *A. ramidus*, which lived about 4.4 million years ago during the early Pliocene, and *A. kadabba*, dated to approximately 5.6 million years ago (late Miocene). *A. ramidus* had a small brain, measuring between 300 and 350 cm3. This is about the same size as the modern bonobo and female common chimpanzee brain; it is somewhat smaller than the brain of australopithecines like Lucy (400 to 550 cm3) and slightly over a fifth the size of the modern *Homo sapiens* brain.
 3.6 Ma
A member of the *Australopithecus afarensis* left human-like footprints on volcanic ash in Laetoli, Kenya (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.
 3.5–3.3 Ma
 3 Ma

Family tree showing the extant hominoids: humans (genus *Homo*), chimpanzees and bonobos (genus *Pan*), gorillas (genus *Gorilla*), orangutans (genus *Pongo*), and gibbons (four genera of the family Hylobatidae: *Hylobates*, *Hoolock*, *Nomascus*, and *Symphalangus*). All except gibbons are hominids.
From Template:Human evolution species chart