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First living beings Date Event 4000 Ma (million years ago) The earliest life appears. Further information: Abiogenesis 3900 Ma Cells resembling prokaryotes appear. This marks the first appearance of photosynthesis and therefore the first occurrence of large quantities of oxygen on the earth. Further information: Cell (biology)#Evolution 2500 Ma First organisms to utilize oxygen. By 2400 Ma, in what is referred to as the Great Oxygenation Event, the pre-oxygen anaerobic forms of life were wiped out by the oxygen consumers. 2100 Ma More complex cells appear: the eukaryotes. Further information: Eukaryote#Origin and evolution 1200 Ma Sexual reproduction evolves, leading to faster evolution.[1] 900 Ma Choanoflagellate The choanoflagellates may look similar to the ancestors of the entire animal kingdom, and in particular they may be the direct ancestors of Sponges.[2][3] 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. 600 Ma It is thought that the earliest multicellular animal was a sponge-like creature. Sponges are among the simplest of animals, with partially differentiated tissues. Sponges (Porifera) are the phylogenetically oldest animal phylum extant today. 580 Ma Animal movement may have started with cnidarians. Almost all cnidarians possess nerves and muscles. Because they are the simplest animals to possess them, their direct ancestors were very probably 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. 550 Ma Flatworm Flatworms are 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. 540 Ma Acorn worms are considered more highly specialised and advanced than other similarly 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.[citation needed] Chordates Date Event 530 Ma Pikaia Pikaia is an iconic ancestor of modern chordates and vertebrates.[4] Other, earlier chordate predecessors include Myllokunmingia fengjiaoa,[5] Haikouella lanceolata,[6] and Haikouichthys ercaicunensis.[7] The lancelet, still living today, retains some characteristics of the primitive chordates. It resembles Pikaia. Conodont Conodonts are a famous type of early (495 Mya and later) chordate fossil; they are the peculiar teeth of an eel-shaped animal characterized by large eyes, fins with fin rays, chevron-shaped muscles and a notochord. The animal is sometimes called a conodont, and sometimes a conodontophore (conodont-bearer) to avoid confusion. 505 Ma Agnatha The first vertebrates appear: the ostracoderms, jawless fish related to present-day lampreys and hagfishes. Haikouichthys and Myllokunmingia are examples of these jawless fish, or Agnatha. (See also prehistoric fish). They were jawless and their internal skeletons were cartilaginous. They lacked the paired (pectoral and pelvic) fins of more advanced fish. They were precursors to the Osteichthyes (bony fish).[8] 480 Ma A Placoderm The Placodermi were prehistoric fishes. Placoderms were some of the first jawed fishes (Gnathostomata), their jaws evolving from the first gill arch.[9] A placoderm's head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked. However, the fossil record indicates that they left no descendents after the end of the Devonian and are less closely related to living bony fishes than sharks are.[citation needed] 410 Ma The first coelacanth appears;[10] 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. Tetrapods Date Event 390 Ma Panderichthys 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.[11] 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. Lungfishes retain some characteristics of the early Tetrapoda. One example is the Queensland Lungfish. 375 Ma Tiktaalik 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. 365 Ma Acanthostega Ichthyostega 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 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.[12] 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. 300 Ma Hylonomus 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. Αlpha keratin first evolves here which is used in claws in modern lizards and birds, and hair in mammals.[13] 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. They have twelve pairs of cranial nerves. Mammals Date Event 256 Ma Phthinosuchus, an early Therapsid Shortly after the appearance of the first reptiles, two branches split off. One branch is the Diapsids, from which come the modern reptiles and birds. The other branch is Synapsida, from which come modern mammals. Both had temporal fenestrae, a pair of holes in their skulls behind the eyes, which were used to increase the space for jaw muscles. Synapsids had one opening on each side, while diapsids had two. 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. 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.[14] 220 Ma Cynognathus One sub-group of therapsids, the cynodonts, evolved more mammal-like characteristics. The jaws of cynodonts resemble modern mammal jaws. It is very likely that this group of animals contains a species which is the direct ancestor of all modern mammals.[15] 220 Ma Repenomamus 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. Monotremes are an egg-laying group of mammals represented amongst modern animals by the platypus and spiny anteaters. 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 gender differentiation through the existence or lack of SRY gene (found in the y-Chromosome) evolved after the monotreme lineage split off. 160 Ma Juramaia sinensis Juramaia sinensis[16] is the earliest known eutherian (placental) mammal fossil. 100 Ma Last common ancestor of mice and humans (base of the clade Euarchontoglires). Primates Date Event 85–65 Ma 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-primates 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.[17] One of the last Plesiadapiformes is Carpolestes simpsoni. It had grasping digits but no forward-facing eyes. 63 Ma Primates diverge into suborders Strepsirrhini (wet-nosed primates) and Haplorrhini (dry-nosed primates). Strepsirrhini contain most of the prosimians; modern examples include the lemurs and lorises. The haplorrhines include the three living groups: prosimian tarsiers, simian monkeys, and apes. One of the earliest haplorrhines is Teilhardina asiatica, 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 descendants had to include fruit in its diet, where Vitamin C could be obtained externally. 30 Ma Aegyptopithecus Haplorrhini splits into infraorders Platyrrhini and Catarrhini. 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 relatively narrow Atlantic ocean (approx. 700 km). Catarrhines mostly stayed in Africa as the two continents drifted apart. Possible early ancestors of catarrhines include Aegyptopithecus and Saadanius. 25 Ma Proconsul 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 Date Event 15 Ma Hominidae (great apes) speciate from the ancestors of the gibbon (lesser apes). 13 Ma Homininae ancestors speciate from the ancestors of the orangutan.[18] 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. It had special adaptations for tree climbing, just as humans and other great apes do: a wide, flat rib cage, a stiff lower spine, flexible wrists, and shoulder blades that lie along its back. 10 Ma The lineage currently represented by humans and the Pan genus (chimpanzees and bonobos) speciates from the ancestors of the gorillas. 7 Ma Sahelanthropus tchadensis Hominina speciate from the ancestors of the chimpanzees. 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. The latest common ancestor lived around the time of Sahelanthropus tchadensis, ca. 7 Ma [4]; S. tchadensis is sometimes claimed to be the last common ancestor of humans and chimpanzees, but there is no way to establish this with any certainty. The earliest known representative from the ancestral human line post-dating the separation with the chimpanzee lines is Orrorin tugenensis (Millennium Man, Kenya; ca. 6 Ma). 4.4 Ma Ardipithecus 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[19] during the early Pliocene, and A. kadabba, dated to approximately 5.6 million years ago[20] (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 like Lucy (~400 to 550 cm3) and slightly over a fifth the size of the modern Homo sapiens brain. Ardipithecus was arboreal, 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 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. 3.6 Ma Australopithecus afarensis Some Australopithecus afarensis left human-like footprints on volcanic ash in Laetoli, Kenya (Northern Tanzania) which provides strong evidence of full-time bipedalism. Australopithecus afarensis lived between 3.9 and 2.9 million years ago. 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 in 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 probably 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. 3.5 Ma Kenyanthropus platyops, a possible ancestor of Homo, emerges from the Australopithecus genus. 3 Ma The bipedal australopithecines (a genus of the Hominina subtribe) evolve in the savannas of Africa being hunted by Dinofelis. Loss of body hair takes place in the period 3-2 Ma, in parallel with the development of full bipedalism. Homo Date Event 2.5 Ma Homo habilis Appearance of Homo. Homo habilis 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 Ma, making it highly unlikely that Homo erectus directly evolved out of Homo habilis. First stone tools, beginning of the Lower Paleolithic. Further information: Homo rudolfensis 1.8 Ma 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 than that of Homo habilis and the teeth are smaller. Other hominid designations such as Homo georgicus, Homo ergaster, Homo pekinensis, Homo heidelbergensis are often put under the umbrella species name of Homo erectus.[21] Starting with Homo georgicus found in what is now the Republic of Georgia dated at 1.8 Ma, the pelvis and backbone grew more human-like and gave H. georgicus the ability to cover very long distances in order to follow herds of other animals. This is the oldest fossil of a hominid found outside of Africa. Control of fire by early humans is achieved 1.5 Ma by 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 Ma. Homo pekinensis first appears in Asia around 700 Ka 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. 1.2 Ma Homo antecessor may be a common ancestor of humans and Neanderthals.[22][23] At present estimate, humans have approximately 20,000–25,000 genes and share 99% of their DNA with the now extinct Neanderthal [24] and 95-99% of their DNA with their closest living evolutionary relative, the chimpanzees.[25][26] The human variant of the FOXP2 gene (linked to the control of speech) has been found to be identical in Neanderthals.[27] It can therefore be deduced that Homo antecessor would also have had the human FOXP2 gene. 600 ka A reconstruction of Homo heidelbergensis Three 1.5 m (5 ft) tall Homo heidelbergensis left footprints in powdery volcanic ash solidified in Italy. Homo heidelbergensis may be a common ancestor of humans and Neanderthals.[28] 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 m (6 ft) and more muscular than modern humans. Beginning of the Middle Paleolithic. 338 ka Y-chromosomal Adam lived in Africa approximately 338,000 years ago, according to a recent study.[29] He is the most recent common ancestor from whom all male human Y chromosomes are descended. 200 ka Homo sapiens sapiens (Pioneer plaque) Omo1, Omo2 (Ethiopia, Omo river) are the earliest fossil evidence for anatomically modern Homo sapiens.[30] 160 ka Homo sapiens (Homo sapiens idaltu) in Ethiopia, Awash River, Herto village, practice mortuary rituals and butcher hippos. Potential earliest evidence of anatomical and behavioral modernity consistent with the continuity hypothesis including use of red ochre and fishing.[31] 150 ka Mitochondrial Eve is a woman who lived in East Africa. She is the most recent female ancestor common to all mitochondrial lineages in humans alive today. Note that there is no evidence of any characteristic or genetic drift that significantly differentiated her from the contemporary social group she lived with at the time. Her ancestors were Homo sapiens as were her contemporaries. 90 ka Appearance of mitochondrial haplogroup L2. 70 ka Behavioral modernity according to the "great leap forward" theory.[32] 60 ka Appearance of mitochondrial haplogroups M and N, which participate in the migration out of Africa. Homo sapiens that leave Africa in this wave start interbreeding with the Neanderthals they encounter.[33][34] 50 ka Migration to South Asia. M168 mutation (carried by all non-African males). Beginning of the Upper Paleolithic. mt-haplogroups U, K. 40 ka Migration to Australia[35] and Europe (Cro-Magnon). 25 ka The independent Neanderthal lineage dies out. Y-Haplogroup R2; mt-haplogroups J, X. 12 ka Beginning of the Mesolithic / Holocene. Y-Haplogroup R1a; mt-haplogroups V, T. Evolution of light skin in Europeans (SLC24A5).[citation needed] Homo floresiensis dies out, leaving Homo sapiens as the only living species of the genus Homo. |
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