| காலம்
<small>(ஆண்டுகளுக்கு&nbsp;முன்பு)</small> | நிகழ்வு |  |
| --- | --- | --- |
| 13,700,000,000 | அண்டத்தின் தோற்றம் |  |
| 4,500,000,000 | பூமியின் தோற்றம். |  |
| 4,000,000,000 | உயிர்களின் தோற்றம் |  |
| 3,900,000,000 | புரோகார்யோட்டுகளை ஒத்த உயிரணுக்களின் தோற்றம் |  |
| 2,500,000,000 | உயிர்வாயுவைப் பயன்படுத்தும் உயிர்கள் தோற்றம். |  |
| 2,100,000,000 | கூடிய சிக்கலான மெய்க்கருவுயிரி தோற்றம். |  |
| 1,200,000,000 | பாலியல் இனப்பெருக்கம், வேகமான படிவளர்ச்சி |  |
| 900,000,000 | ]]
The choanoflagellates may look similar to the மூதாதை of the entire விலங்கு 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 விலங்குs may have looked like.  
They live in colonies, and show a primitive level of cellular specialization for different tasks. |  |
| 600,000,000 | முதல் வகை பல் உயிரணு உயிரனம் கடல்பஞ்சு போன்றதென்று கருதப்படுகிறது.  
Sponges (பஞ்சுயிரி) are the phylogenetically oldest விலங்கு தொகுதி (உயிரியல்) extant today. |  |
| 580 Ma | The movement of all animals may have started with நிடேரியா. Almost all cnidarians possess நரம்பு and தசைs and, because they are the simplest விலங்குs to possess it, their direct மூதாதைs 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. |  |
| 550 Ma | ]] மூளையுடன் தோன்றிய முதல் ஊயிரினம் தட்டைப்புழுவாகும். 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 சுற்றோட்டத் தொகுதி with a இதயம் that also functions as a சிறுநீரகம். Acorn worms have a செவுள்-like structure used for மூச்சுவிடல்ing, a structure similar to that of primitive fish. Acorn worms are thus sometimes said to be a link between முள்ளந்தண்டுளி and முதுகெலும்பிலி. |  |
| 530 Ma | ]]
The earliest known ancestor of the முதுகுநாணி is Pikaia. It is the first known விலங்கு with a முதுகுநாண். Pikaia is believed to be the ancestor of all முதுகுநாணி and முள்ளந்தண்டுளி.  
The Lancelet, still living today, retains some characteristics of the primitive முதுகுநாணிs. It resembles Pikaia  
Other earliest known chordate-like fossils is from a conodonts a "eel-shaped animal of 4-20&nbsp;cm (1½-8&nbsp;in) long" with a pair of huge eyes at the head end were and a complex basket of teeth. |  |
| 505 Ma | ]]  
The first முள்ளந்தண்டுளிs 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 மீன். They were the Precursors to the bony fish. |  |
| 480 Ma | ]]
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. |  |
| 400 Ma | First *Coelacanth* appears; 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 வாழும் தொல்லுயிர் எச்சம். |  |
| 375 Ma | *Tiktaalik* is a genus of sarcopterygian (lobe-finned) fishes from the late Devonian with many tetrapod-like features. |  |
| 365 Ma | ]]
Some fresh water lobe-finned மீன் (Sarcopterygii) develop legs and give rise to the நாற்காலி (உயிரியல்).  
The first tetrapods evolved in shallow and swampy நன்னீர் habitats.  
Primitive tetrapods developed from a lobe-finned fish (an "osteolepid Sarcopterygian"), with a two-lobed மனித மூளை 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 எலும்புs. 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.  
Panderichthys is a 90–130&nbsp;cm (35–50&nbsp;in) long மீன் from the Late டெவோனியக் காலம். It has a large நாற்காலி (உயிரியல்)-like தலை. Panderichthys exhibits features transitional between lobe-finned fishes and early நாற்காலி (உயிரியல்).  
நுரையீரல்மீன்es retain some characteristics of the early நாற்காலி (உயிரியல்)s. One example is the Australian Lungfish. |  |
| 315 Ma | ]]
]]
Acanthostega is an extinct நீர்நில வாழ்வன, among the first animals to have recognizable limbs. It is a candidate for being one of the first முள்ளந்தண்டுளிs 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 நுரையீரல் and செவுள், also indicating it was a link between lobe-finned fish and terrestrial vertebrates.  
Ichthyostega is an early நாற்காலி (உயிரியல்). Being one of the first animals with legs, arms, and finger bones, Ichthyostega is seen as a hybrid between a மீன் and an நீர்நில வாழ்வனn. Ichthyostega' had legs but its limbs probably weren't used for நடத்தல், they may have spent very brief periods out of water and would have used their legs to paw their way through the mud.  
நீர்நில வாழ்வன were the first four-legged animals to develop நுரையீரல்.  
நீர்நில வாழ்வனns living today still retain many characteristics of the early நாற்காலி (உயிரியல்). |  |
| 300 Ma | ]]
From amphibians came the first reptiles: Hylonomus is the earliest known ஊர்வன. It was 20&nbsp;cm (8&nbsp;in) long (including the tail) and probably would have looked rather similar to modern பல்லியோந்திகள்s. It had small sharp teeth and probably ate millipedes and early பூச்சிs. It is a precursor of later amniotes and mammal-like reptiles.  
Evolution of the amniotic egg gives rise to the Amniota, ஊர்வனs 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 system, compared to நீர்நில வாழ்வன. They have twelve pairs of cranial nerves. |  |
| 256 Ma | , an early Therapsid]]
Shortly after the appearance of the first ஊர்வனs, two branches split off. One branch is the Diapsida from which come the modern ஊர்வனs. The other branch is Synapsida which had தலையோடு, a pair of holes in their skulls behind the eyes, which were used to increase the space for jaw muscles.  
The earliest **mammal-like reptiles** are the pelycosaurs. The pelycosaurs were the first animals to have temporal fenestra. Pelycosaurs are not Therapsida but soon they gave rise to them. The Therapsida are the direct ancestor of பாலூட்டி.  
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. |  |
| 220 Ma | One sub-group of therapsids, the cynodonts evolved more mammal-like characteristics.
The jaws of cynodonts resemble modern mammal jaws. It is very likely this group of animals contains a species which is the direct ancestor of all modern mammals. |  |
| 220 Ma | '']]