Being similar living material and genetic code; similar life processes, namely uptake of matter and energy, metabolic activities, energy transformation, protein synthesis, reproduction, differentiation and development, respond to stimuli; maintain homeostasis; once the living cells were formed, they could gather to form colonies, in which cell differentiation led to multi-cellular organism. The later evolved into all the existing forms of life by gradual modification. All organisms have and obey the same principles of heredity and evolution. These similarities prove that all organisms have evolved from a common primitive ancestor. The doctrine of organic evolution states that the present complex organisms have been produced by gradual change in the earlier simpler forms of life over the ages. Evolution may briefly be defined as descent with “modification”.

The earliest life appears around 4000 mya. Cells resembling prokaryotes appear around 3900 mya. This marks the first appearance of photosynthesis and therefore the first occurrence of large quantities of oxygen on the earth. First organisms to utilize oxygen around 2500 mya and by 2400 mya, in what is referred to as the great oxygenation event, the pre-oxygen anaerobic forms of life were wiped out by the oxygen consumers. More complex cells appear the eukaryotes around 2100 mya. Sexual reproduction evolves, around 1200 mya, which leads to faster evolution. The chonoflagellates 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 ancestors of all animals may have looked like.

They live in colonies, and show a primitive leave of cellular specialization for different tasks around 900 mya. It is thought that the earliest multicellular animal was a sponge-like creature. Sponges are among the simplest of animals, around 600 mya with partially differentiated tissues. Sponges (Porifera) are the phylogenetically oldest animal phylum extant today.

Animal movement may have started around 580 mya with cnidarians. Almost all enidarians possess nerves and muscles. Because they are the simplest animals to possess them, their direct ancestors were very likely the first animals to use nerves and muscles together. Cnidarians are also the first animals with an actual body of definite form and shape. They have radial symmetry. The first eyes evolved at this time.

Flatworms are around 550 mya and the earliest animals to have a brain, and the simplest animals alive to have bilateral symmetry. They are also the simplest animals with organs that form from three germ layers.

Acorn worms around 540 mya are considered more highly specialized and advanced than other similarity shaped worm-like creatures. They have 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 are thus sometimes said to be a link between vertebrates and invertebrates.

Pikaia is an iconic ancestor of modern chordates and vertebrates around 530 mya. Other, earlier chordate predecessors include Myllokunmingiafengjiaoa, Haikouella lanceolata, and Haikoucella lanceolata, and Haikouichthys ercaicunenis. The lancelet, still living today, retains some characteristics of the primitive chordates. It resembles Pikaia.

Conodonts are a famous type of early (495 mya and later) chordate fossil; they are the peculiar teeth of a cell-shaped animal characterized by large eyes, fins with fin rays, chevron-shaped muscles and a notochord. The animals are sometimes called a conodont, and sometimes a conodontophore (conodont bearer) to avoid confusion. Around 505 mya Agnatha, the first vertebrates appear; the ostracodernis, jaw less fish related to present-day lampreys and hagfishes. Haikouichthys and Myllokunmingia are examples of these jawless fish, or Agnatha. They were jawless and their internal skeletons were cartilaginous. They locked the paired (pectoral and pelvic) fins of more advanced fish.

They were precursors to the osteichthyes (bonny fish). The Placodermi around 480 mya 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 armored plates and the rest of the body was sealed or naked.

The first coelacanth appears around 410 mya; this order of animals had been thought to have no extant members until living specimens were discovered in 1938. It is often referred to as a living fossil.

Panderichthys- Some fresh water lobe-finned fish (Sarcopterygii) develop legs and gives rise to Tetrapoda around 390 mya. The first tetra pods evolved in swallow 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 adoptions of fins with fleshy bases and bones. The “living fossil” coelacanth is a related lobe-finned fish without these swallow-water adaptations. These fishes used their fins as paddles in swallow-water habitats choked with plants and detritus.

The universal tetra pod 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 tetra pods living in swallow water. Panderichthys is a 90-130 cm (30-35 inch) 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 tetra pods. Track way impressions made by something that resembles Ichthyostega’s limbs were formed 390 MYA in polish marine tidal sediments. This suggests tetra pod evolution is older than the dated fossils of Panderichthys through to Ichthyostega. Lungfishes retain some characteristics of the early Tetrapoda. One example is the Queenland lungfish.

Tiktaalik, around 375 mya, is a genus of sarcopterygian (lobed-finned) fishes from the late Devonian with many tetra pod-like features. It shows a clear link between Panderichthys and Aeanthostega.

Aeanthostega around 365 mya, 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 tetra pod. 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 weren’t 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. Amphibians living today still retain many characteristics of the early tetrapods.

Hylonomus – from amphibians came the first reptiles around 300 mya. 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 mammoth like reptiles. A-keratin first evolves here which is used in claws in modern lizards and birds, and hair in mammals. Evolution of the amniotic egg gives rise to the Amniota, reptiles that can reproduces 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. They have twelve pairs of cranial nerves.

Phthinosuchus, an early Therapsid – shortly after appearance of the first reptiles, two branches split off around 256 mya. One branch is the Diapsids, from which come the modern reptiles. The other branch is Synapsida, which had temporal fenestra, a pair of holes in their skulls behind the eyes, which were used to increase the space for jaw muscles. The earliest mammoth-like reptiles are the Pelycosaurs. The Pelycosaurs were the first animals to have temporal fenestra. Pelycosaurs are not Therapsids but soon they gave rise to them. The Therapsida were the direct ancestor of mammals. 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 evolved around 220 mya, more mammal-like characteristics. The jaws of cynodonts resemble modern mammal jaws. It is likely this group of animals contains a species, which is the direct ancestor of all modern mammals.

Repenomamus – from Eucynodontia (Cynodonts) came the first mammals around 220 mya. Most early mammals were small and 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, milk glands for their young. The neocortex region of the brain first evolved in mammals and thus is unique to them.

Eomaia Scansoria, a eutherian mammal, leads to the formation of modern placental mammals around 125 mya. It looks like dormouse, climbing small shrubs in Liaoning, China.

Monotremes are an egg-laying group of mammals represented amongst modern animals by the platypus and spimy anteaters that split away from the eutherian mammals. Recent genome sequencing of the platypus indicates that its sex genes are closer to that of birds. Thus it can be inferred that the first mammals to gain single paired sex determining genes (XX or XY) evolved at or after this point within the eutherian group.

Common genetic ancestor of mice and humans (base of the clade Euarchontoglires) appear around 100 mya.

Around 65-85 mya Plesiadapis, Carpolestes simpsoni – A group of small nocturnal and arboreal, insect-eating mammals called the Euarchonta begins a speciation that will lead to the primate, treeshrew and flying lemur orders. The Primatomorpha is a subdivision of Euarchonta that includes the primates and the proto-primate plesiadapiformes. One of the early proto-primate is Plesiadapis. Plesiadapis still had claws and the eyes located on each side of the head. Because of this they were faster on the ground than on the top of the trees, but they began to spend long times on lower branches of trees, feeding on fruits and leaves. The Plesiadapiformes very likely contain the species, which is the ancestor of all primates. One of the last Plesiadapiformes is Carpolestes simpsoni. It had grasping digits but no forward facing eyes.

Primates diverge into suborders Strepsirrhini (wet-nosed primates) and Haplorrhini (dry-nosed primates) around 40 mya. Strepsirrhini contain most of the prosimians; modern examples include the lemurs and lorises. The haplorrhines include the three living groups; prosimiantarsiers, simian monkeys, and apes. One of the earliest haplorrhines is Teilhardiana asistica, a mouse sized, diurnal creature with small eyes. The Haplorrhini metabolism lost the ability to make its own vitamin C. This means that it and all its descendents had to include fruit in its diet, where vitamin c could be obtained externally.

Aegyptopithecus – Haplorrhini splits into infra orders platyrrhini and catarrhini around 30 mya. Platyrrhines, new world monkeys, have prehensile tails and males are color blind. They may have migrated to South America on a raft of vegetation across the Atlantic Ocean (4500 Km, 2800 mi).  Catarrhines mostly stayed in Africa as the two continents drifted apart. One ancestor of catarrhines might be Aegyptopithecus.

Proconsul- Catarrhini splits into two superfamilies; old world monkeys (Cercopithecoidea) and  apes (Hominoidal) around 25 mya. 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 includes thin tooth enamel, a light build with a narrow chest and short forelimbs, and an arboreal quadrupedal life style. Its ape-like features are its lack of a tail, ape-like elbows, and slightly larger brain relative to body size.

Hominidae (great apes) speciate from the ancestors of the gibbon (lesser apes) around 15 mya.

Homininae ancestors’ speciate from the ancestors of orangutan around 13 mya. Pierologithecus catalaunicus is believed to be a common ancestor of humans and the great apes or 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 lie along its back.

Hominini speciate from the ancestors of the gorillas around 10 mya.

Sahelanthropus tchadensis – Hominina speciate from the ancestors of the chimpanzes around 7 mya. The latest common ancestor lived around the time of sahelanthropus tchadensis, s.tchadensis is sometimes claimed to be the last common ancestor of humans and chimpanzees, but this is disputed. The earliest known human ancestor post-dating the separation of the human and the chimpanzee lines is orrorintugenesis (Millennium Man, Kenya; Ca 6 mya).  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 precursor of speech.

Ardipithecus exists around 4.4 mya is a very early hominin genus (subfamily Homininae). 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 mya. (Late Miocene). A. Ramidus had a small brain, measuring between 300 and 350 cm³. This is about the same size as modern bonobo and female common chimpanzee brain, but much smaller than the brain of Australopithecines likes Lucy (~400 to 550 cm³) and slightly over a fifth the size of modern Homo sapiens brain. Ardipithecus was aboreal, meaning it lived largely in the forest where it competed with other forest animals for food, including the contemporary ancestor for the chimpanzees. Ardipithecus was likely 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.

Australopithecus afarensis – Some Australopithecus afarensis around 3.6 mya left human like footprints on volcanic ash in Lactoil, Kenya (Northern Tanzania) which provides strong evidence of full-time bipedalism. Australopithecus afarensis lived between 3.9 and 2.9 mya. 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 has reduced canines and molars, although they are still relatively larger than modern humans.  A.afarensis also has a relatively small brain size (~380-430 cm³) and a prognathic (i.e. projecting anteriorly) face. Australopithecines have been found in savannah environments and likely increased its diet to include meat from scavenging opportunities. An analysis of Australopithecus africanus lower vertebrae suggests that females had changes to support bipedalism even while pregnant.

Kenyanthropus platyops, a possible ancestor of Homo, emerges from the Australopithecus genus around 3.5 mya.

The bipedal australopithecines (a genus of the Hominina subtribe) evolve around 3 mya in the savannas of Africa being hunted by the Dionofelis. Loss of body hair takes place in the period 3.2 mya, in parallel with the development of full bipedalism.

Appearance of Homo is around 205 mya. Homo habilus is thought to be the ancestor of the lankier and more sophisticated Homo ergaster. Lived side by side with Homo-erectus until at least 1.44 mya, making it highly unlikely that Homo erectus directly evolved out Homo habilis. First tools were beginning of the lower Palaeolithic.

Homo rudolfensis appears around 1.8 mya. A reconstruction of Homo erectus – 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. Its forehead is less sloping and the teeth are smaller. Other hominid designations such as Homo georgicus, Homo ergaster, Homo pikinensis, Homo heidelbergensis are often put under the umbrella species name of Homo erectus. Starting with Homo georgicus found in what is now the Republic of Georgia dated 1.8 mya, the pelvis and backbone grew more human-like and gave H.georgicus the ability cover very long distances in order to follow herds of other animals. This is the oldest fossil of a hominid found (so far) outside of Africa. Control of fire by early humans is achieved 1.5 mya Homo ergaster. Homo ergaster reaches a height of around 1.9 metres (6.2 ft). Evolution of dark skin, which is linked to the loss of body hair in human ancestors, is complete by 1.2 mya. Homo pekinensis first appears in Asia around 700 kya but according to the theory of a recent African origin of modern humans, they could not be human ancestors, but rather were just a cousin offshoot species from Homo ergaster. Homo heidelbergensis was a very large hominid that had a more advanced complement of cutting tools and may have hunted big game such as horses.

Homo antecessor appears in 1.2 mya, is the common genetic ancestor of humans and Neanderthal. At present estimates, humans have approximately 20000-25000 genes and share 99% of their DNA with the now extinct Neanderthal and 95-99% of their closet living evolutionary relative, the chimpanzees. The human variant of the FoxP2 gene (linked to the control of speech) has been found to be identical in Neanderthals. It can therefore be deducted that Homo antecessor would also have had the human FOXP2 gene.

A reconstruction of Homo heidelbergensis around 600 kya – there 1.5 mt (5ft) tall Homo heidelbergensis left foot prints in powdery volcanic ash solidified in Italy. Homo heidelbergensis is the common ancestor of both Homo neanderthalensis and Homo Sapiens. It is morphologically very similar to Homo erectus, but Homo heidelbergensis had a larger brain-case about 93% the size of that of Homo Sapiens. The holotype of the species was tall, 1.8 mt (6ft) and more mascular than modern humans, beginning of the middle Paleolithic.