Timeline of human evolution

For more about the Origins of cells, see : Origins of cells

The symbiotic mitochondria, appear similar to today's Rickettsia.

The origin of the eukaryotic cell was a milestone in the evolution of life. Their higher level of organizational complexity has permitted the development of truly multicellular organisms. Without eukaryotes, the world would lack mammals, birds, fish, invertebrates, mushrooms, plants, and complex single-celled organisms.

The choanoflagellates are considered ancestors of the animal kingdom, and in particular may be the direct ancestors of sponges.The choanocytes cells of sponges have the same basic structure as choanoflagellates. Collared cells are occasionally found in a few other animal groups, such as flatworms. Comparisons of DNA sequences support a close affiliation between choanoflagellates and animals.

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 cell specialization for different tasks. the similarities between Proterospongia and sponges are strong evidence for the close relationship between choanoflagellates and animals.

Proterospongia is not the direct ancestor of sponges, but it must look like what the ancestor of sponges all multicellular animals may have looked like. (the connecting link between protozoa and metazoans.)

It is thought that the earliest multicellular life on Earth was a sponge-like creature. Sponges are among the simplest of animals, with partially differentiated tissues but without muscles, nerves, or internal organs.

Sponges Porifera are the phylogenetically oldest animal phylum extant today.

In some ways they are closer to being cell colonies than multicellular organisms.

The earliest known ancestor of the chordates is Pikaia.It the first known animal with a notochord.Pikaia is believed to be the ancestor of all chordates and vertebrates.

Lancelet , that still lives today , retain some characteristics of the primitive chordates. It resembles Pikaia

Other earliest known chordate-like fossils is from a conodonts a "eel-shaped animal of 4-20 cm long" with a pair of huge eyes at the head end were and a complex basket of teeth.

Agnatha, the Ostracoderm was the first fishes (first vertebrates) Precursors to the bony fish.they was jawless such as arandaspis.Their internal skeletons were cartilaginous . They lacked the paired (pectoral and pelvic) fins of more advanced fish.

The Placodermi were prehistoric fishes . Placoderms were the first of the jawed fishes, their jaws evolving from the first of their gill arches.Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked.

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. These 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.

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 to come onto land. The limbs could not support the animal's weight.

The fish had both lungs and gills, also indicating it was a link between lobe-finned fish and terrestrial vertebrates.

Amphibia were the first four-legged animals to develop lungs. During the Carboniferous period they also developed the ability to walk on land to avoid aquatic competition and predation while allowing them to travel from water source to water source , and move out onto land, probably to hunt insects.

The earliest tetrapods were not terrestrial. The earliest confirmed terrestrial forms are known from the early Carboniferous. Still, they may have spent very brief periods out of water and would have used their legs to paw their way through the mud.

Amphibians still retain many characteristics of the early tetrapods.

Early tetrapods : Temnospondyl, Anthrachosaur,Lepospondyl ,Ichthyostega , Acanthostega and Pederpes finneyae

Therapsida are the direct ancestor of mammals they are often called mammal-like reptiles.

The earliest mammal-like reptilian are the pelycosaurs. The pelycosaurs was the first animals to have temporal fenestra.pelycosaurs are not Therapsida but soon they gave rise to them. The therapsids have temporal fenestrae larger and more mammal-like than pelycosaurs , their teeth show more serial differentiation; and later forms had evolved a secondary palate. A secondary palate enables the animal to eat and breathe at the same time and is a sign of a more active, perhaps warm-blooded, way of life.

One sub-group of therapsids, the cynodonts have evolved more mammal-like characteristics. The jaws of cynodonts resemble modern mammal jaws more closely and their teeth are multi-cusped and differentiated down the jaw. Cynodonts are the ancestors of all modern mammals.

From synapsids came the first mammal precursors, therapsids, and more specifically the eucynodonts.

Most early mammals were small and shrew-like animals that fed on insects. Constant body temperature. All mammals have milk glands for their young.

The earliest mammals include:

• Eozostrodon: Triassic and Jurassic
• Deltatheridium: Cretaceous
• Jeholodens: mid-Cretaceous
• Megazostrodon: late Triassic and early Jurassic
• Triconodont: Triassic to Cretaceous
• Zalambdalestes: late Cretaceous

Eomaia scansoria, a eutherian mammal, leads to the formation of modern placental mammals. It looks like modern dormouse, climbing small shrubs in Liaoning, China.

One of the last Plesiadapiformes, Carpolestes simpsoni, appears. It had grasping digits but no forward facing eyes.

Haplorrhini splits into infraorders Platyrrhini and Catarrhini. Catarrhines mostly stayed in Africa as the two continents drifted apart. One ancestor of catarrhines might be Aegyptopithecus. Other ancient catarrhines include Bugtipithecus inexpectans, Phileosimias kamali and Phileosimias brahuiorum, which are all similar to today's lemurs.

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

Pierolapithecus had special adaptations for tree climbing, just as humans and other great apes do: a wide, flat ribcage, a stiff lower spine, flexible wrists, and shoulder blades that lay along its back.

Human ancestors speciate from the ancestors of the chimpanzees. The latest common ancestor is Sahelanthropus tchadensis .The earliest in the human branch is Orrorin tugenensis (Millennium Man, Kenya). Both chimpanzees and humans have 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 precursor of speech.

Homo habilis is thought to be the ancestor of the lankier and more sophisticated, Homo ergaster, which in turn gave rise to the more human appearing species, Homo erectus. There is debate over whether H. habilis is a direct human ancestor, and over how many known fossils are properly attributed to the species.

see:Homo rudolfensis

Homo erectus evolves in Africa . Homo erectus would bear a striking resemblance to modern humans, but had a brain about 74 percent of the size of modern man. forehead is less sloping and the teeth are smaller .

It is believed to be an ancestor of modern humans (with Homo heidelbergensis usually treated as an intermediary step).

It is morphologically very similar Homo erectus but Homo heidelbergensis had a larger brain-case, about 93% of Homo sapiens size.The species was tall, 1.8 m (6 ft.) on average, and more muscular than modern humans.

The first anatomically modern humans (Homo sapiens) appear in Africa some time before this, they evolved from Homo heidelbergensis.

At present estimate, humans have approximately 20,000–25,000 genes and share 98.5% of their DNA with their closest living evolutionary relative, the bonobos. [1]

Homo sapiens skin is relatively hairless in comparison to other primates. The skin colour of contemporary humans can range from very dark brown to very pale pink. It is geographically stratified and in general correlates with the environmental level of UV. Human skin and hair colour is controlled in part by the MC1R gene. For example, the red hair and pale skin of some Europeans is the result of mutations in MC1R. Human skin has a capacity to darken (sun tanning) in response to UV exposure. Variation in the ability to sun tan is also controlled in part by MC1R.