What Animals Were Around 20 Million Years Ago

New Scientist Default Image — Galapagos tortoises are the product of over three billion years of development

Andy Rouse / Getty

There are all sorts of ways to reconstruct the history of life on Globe. Pinning downwardly when specific events occurred is often tricky, though. For this, biologists depend mainly on dating the rocks in which fossils are establish, and by looking at the "molecular clocks" in the DNA of living organisms.

There are bug with each of these methods. The fossil record is like a pic with most of the frames cut out. Considering it is so incomplete, it tin be difficult to constitute exactly when particular evolutionary changes happened.

Modernistic genetics allows scientists to mensurate how unlike species are from each other at a molecular level, and thus to approximate how much time has passed since a single lineage divide into dissimilar species. Misreckoning factors rack upward for species that are very distantly related, making the earlier dates more uncertain.

These difficulties mean that the dates in the timeline should be taken as approximate. As a general rule, they become more than uncertain the further back along the geological timescale we expect. Dates that are very uncertain are marked with a question mark.

3.eight billion years ago?

This is our current "best estimate" for the commencement of life on Earth. It is distinctly possible that this date will change every bit more evidence comes to light. The first life may have developed in undersea alkali metal vents, and was probably based on RNA rather than DNA.

At some point far back in time, a mutual ancestor gave ascent to two principal groups of life: bacteria and archaea.

How this happened, when, and in what social club the different groups separate, is nevertheless uncertain.

3.v billion years ago

The oldest fossils of single-celled organisms engagement from this fourth dimension.

iii.46 billion years agone

Some unmarried-celled organisms may be feeding on methane past this time.

3.4 billion years ago

Stone formations in Western Commonwealth of australia, that some researchers claim are fossilised microbes, appointment from this period.

three billion years agone

Viruses are present past this fourth dimension, just they may be as old as life itself.

ii.four billion years ago

The "keen oxidation event". Supposedly, the poisonous waste matter produced by photosynthetic cyanobacteria – oxygen – starts to build upwardly in the atmosphere. Dissolved oxygen makes the atomic number 26 in the oceans "rust" and sink to the seafloor, forming striking banded fe formations.

Recently, though, some researchers have challenged this idea. They think cyanobacteria only evolved later, and that other bacteria oxidised the iron in the absenteeism of oxygen.

However others call up that cyanobacteria began pumping out oxygen equally early as 2.1 billion years ago, only that oxygen began to accumulate merely due to some other factor, possibly a turn down in methane-producing bacteria. Methane reacts with oxygen, removing it from the atmosphere, so fewer methane-belching leaner would let oxygen to build up.

2.3 billion years ago

Earth freezes over in what may have been the first "snowball World", possibly as a result of a lack of volcanic activity. When the ice eventually melts, information technology indirectly leads to more oxygen being released into the atmosphere.

2.15 billion years agone

First undisputed fossil prove of blue-green alga, and of photosynthesis: the power to take in sunlight and carbon dioxide, and obtain free energy, releasing oxygen as a by-product.

There is some prove for an earlier date for the beginning of photosynthesis, but it has been called into question.

2 billion years ago?

Eukaryotic cells – cells with internal "organs" (known as organelles) – come into existence. One key organelle is the nucleus: the control centre of the jail cell, in which the genes are stored in the form of DNA.

Eukaryotic cells evolved when 1 simple cell engulfed another, and the two lived together, more or less amicably – an example of "endosymbiosis". The engulfed bacteria eventually get mitochondria, which provide eukaryotic cells with energy. The last common ancestor of all eukaryotic cells had mitochondria – and had also developed sexual reproduction.

Later, eukaryotic cells engulfed photosynthetic bacteria and formed a symbiotic human relationship with them. The engulfed bacteria evolved into chloroplasts: the organelles that requite green plants their colour and let them to extract free energy from sunlight.

Different lineages of eukaryotic cells acquired chloroplasts in this way on at least three separate occasions, and one of the resulting cell lines went on to evolve into all green algae and green plants.

1.5 billion years ago?

The eukaryotes divide into 3 groups: the ancestors of modern plants, fungi and animals split into split lineages, and evolve separately. We do not know in what gild the three groups bankrupt with each other. At this time they were probably all yet unmarried-celled organisms.

900 million years ago?

The commencement multicellular life develops around this time.

It is unclear exactly how or why this happens, but one possibility is that single-celled organisms go through a phase similar to that of modern choanoflagellates: single-celled creatures that sometimes form colonies consisting of many individuals. Of all the unmarried-celled organisms known to exist, choanoflagellates are the near closely related to multicellular animals, lending support to this theory.

800 million years agone

The early multicellular animals undergo their get-go splits. Showtime they divide into, substantially, the sponges and everything else – the latter beingness more formally known equally the Eumetazoa.

Around 20 meg years later, a modest group called the placozoa breaks away from the balance of the Eumetazoa. Placozoa are thin plate-similar creatures about 1 millimetre across, and consist of only 3 layers of cells. It has been suggested that they may actually be the last common ancestor of all the animals.

770 meg years ago

The planet freezes once again in another "snowball Earth".

730 million years agone

The comb jellies (ctenophores) split from the other multicellular animals. Like the cnidarians that will soon follow, they rely on h2o flowing through their body cavities to acquire oxygen and food.

680 one thousand thousand years ago

The antecedent of cnidarians (jellyfish and their relatives) breaks abroad from the other animals – though there is as nevertheless no fossil bear witness of what information technology looks like.

630 million years agone

Effectually this time, some animals evolve bilateral symmetry for the first time: that is, they at present have a defined meridian and lesser, also as a front and back.

Piddling is known nigh how this happened. However, small worms chosen Acoela may be the closest surviving relatives of the get-go ever bilateral beast. It seems likely that the first bilateral animal was a kind of worm. Vernanimalcula guizhouena, which dates from around 600 million years agone, may be the earliest bilateral animal establish in the fossil record.

590 million years ago

The Bilateria, those animals with bilateral symmetry, undergo a profound evolutionary split. They divide into the protostomes and deuterostomes.

The deuterostomes eventually include all the vertebrates, plus an outlier group called the Ambulacraria. The protostomes get all the arthropods (insects, spiders, venereal, shrimp so forth), diverse types of worm, and the microscopic rotifers.

Neither may seem like an obvious "group", but in fact the 2 can be distinguished by the manner their embryos develop. The first pigsty that the embryo acquires, the blastopore, forms the anus in deuterostomes, but in protostomes it forms the mouth.

580 one thousand thousand years ago

The earliest known fossils of cnidarians, the group that includes jellyfish, ocean anemones and corals, date to around this time – though the fossil evidence has been disputed.

575 1000000 years agone

Strange life forms known as the Ediacarans announced around this time and persist for about 33 million years.

570 meg years agone

A modest group breaks abroad from the chief group of deuterostomes, known equally the Ambulacraria. This group eventually becomes the echinoderms (starfish, brittle stars and their relatives) and two worm-like families called the hemichordates and Xenoturbellida.

Another echinoderm, the ocean lily, is thought to be the "missing link" between vertebrates (animals with backbones) and invertebrates (animals without backbones), a split that occurred around this fourth dimension.

565 million years agone

Fossilised fauna trails advise that some animals are moving under their own ability.

540 million years ago

As the first chordates – animals that take a backbone, or at least a archaic version of information technology – emerge among the deuterostomes, a surprising cousin branches off.

The sea squirts (tunicates) begin their history every bit tadpole-like chordates, merely metamorphose partway through their lives into bottom-dwelling filter feeders that look rather like a pocketbook of seawater anchored to a rock. Their larvae nonetheless look like tadpoles today, revealing their close relationship to backboned animals.

535 meg years agone

The Cambrian explosion begins, with many new body layouts appearing on the scene – though the seeming rapidity of the advent of new life forms may simply be an illusion caused past a lack of older fossils.

530 meg years ago

The get-go truthful vertebrate – an animal with a courage – appears. It probably evolves from a jawless fish that has a notochord, a stiff rod of cartilage, instead of a truthful backbone. The first vertebrate is probably quite like a lamprey, hagfish or lancelet.

Around the aforementioned fourth dimension, the first clear fossils of trilobites appear. These invertebrates, which expect like oversized woodlice and grow to seventy centimetres in length, proliferate in the oceans for the adjacent 200 one thousand thousand years.

520 million years agone

Conodonts, another contender for the title of "earliest vertebrate", appear. They probably look like eels.

500 one thousand thousand years ago

Fossil evidence shows that animals were exploring the land at this time. The first animals to practise so were probably euthycarcinoids – thought to exist the missing link between insects and crustaceans. Nectocaris pteryx, thought to be the oldest known antecedent of the cephalopods – the group that includes squid – lives around this time.

489 1000000 years ago

The Great Ordovician Biodiversification Event begins, leading to a slap-up increase in multifariousness. Within each of the major groups of animals and plants, many new varieties appear.

465 million years ago

Plants begin colonising the land.

460 million years ago

Fish divide into two major groups: the bony fish and cartilaginous fish. The cartilaginous fish, as the name implies, have skeletons fabricated of cartilage rather than the harder bone. They eventually include all the sharks, skates and rays.

440 1000000 years ago

The bony fish divide into their two major groups: the lobe-finned fish with basic in their fleshy fins, and the ray-finned fish. The lobe-finned fish eventually give rising to amphibians, reptiles, birds and mammals. The ray-finned fish thrive, and requite rise to most fish species living today.

The common ancestor of lobe-finned and ray-finned fish probably has simple sacs that function as primitive lungs, assuasive it to gulp air when oxygen levels in the water autumn too depression. In ray-finned fish, these sacs evolve into the swim bladder, which is used for controlling buoyancy.

425 meg years ago

The coelacanth, one of the nearly famous "living fossils" – species that have apparently not changed for millions of years – splits from the rest of the lobe-finned fish.

417 million years ago

Lungfish, another legendary living fossil, follow the coelacanth by splitting from the other lobe-finned fish. Although they are unambiguously fish, complete with gills, lungfish have a pair of relatively sophisticated lungs, which are divided into numerous smaller air sacs to increase their surface area. These allow them to breathe out of water and thus to survive when the ponds they alive in dry out.

400 1000000 years ago

The oldest known insect lives effectually this time. Some plants evolve woody stems.

397 million years ago

The first four-legged animals, or tetrapods, evolve from intermediate species such as Tiktaalik, probably in shallow freshwater habitats.

The tetrapods go on to conquer the land, and give ascent to all amphibians, reptiles, birds and mammals.

385 1000000 years agone

The oldest fossilised tree dates from this menstruation.

375 million years ago

Tiktaalik, an intermediate between fish and 4-legged land animals, lives effectually this time. The fleshy fins of its lungfish ancestors are evolving into limbs.

340 1000000 years agone

The first major split occurs in the tetrapods, with the amphibians branching off from the others.

310 one thousand thousand years agone

Within the remaining tetrapods, the sauropsids and synapsids split from i another. The sauropsids include all the modern reptiles, plus the dinosaurs and birds. The starting time synapsids are also reptiles, only have distinctive jaws. They are sometimes called "mammal-like reptiles", and eventually evolve into the mammals.

320 to 250 million years ago

The pelycosaurs, the kickoff major group of synapsid animals, dominate the land. The about famous instance is Dimetrodon, a big predatory "reptile" with a sail on its back. Despite appearances, Dimetrodon is not a dinosaur.

275 to 100 million years agone

The therapsids, close cousins of the pelycosaurs, evolve aslope them and eventually supplant them. The therapsids survive until the early Cretaceous, 100 million years agone. Well earlier that, a group of them called the cynodonts develops domestic dog-similar teeth and eventually evolves into the outset mammals.

250 million years agone

The Permian menstruation ends with the greatest mass extinction in World'south history, wiping out great swathes of species, including the final of the trilobites.

Equally the ecosystem recovers, it undergoes a central shift. Whereas before the synapsids (start the pelycosaurs, then the therapsids) dominated, the sauropsids now take over – nearly famously, in the class of dinosaurs. The ancestors of mammals survive equally modest, nocturnal creatures.

In the oceans, the ammonites, cousins of the modern nautilus and octopus, evolve around this time. Several groups of reptiles colonise the seas, developing into the slap-up marine reptiles of the dinosaur era.

210 million years ago

Bird-similar footprints and a badly-preserved fossil chosen Protoavis suggest that some early dinosaurs are already evolving into birds at this time. This claim remains controversial.

200 million years agone

As the Triassic period comes to an end, some other mass extinction strikes, paving the way for the dinosaurs to take over from their sauropsid cousins.

Around the aforementioned time, proto-mammals evolve warm-bloodedness – the power to maintain their internal temperature, regardless of the external conditions.

180 million years ago

The offset separate occurs in the early mammal population. The monotremes, a group of mammals that lay eggs rather than giving birth to live young, break autonomously from the others. Few monotremes survive today: they include the duck-billed platypus and the echidnas.

168 one thousand thousand years ago

A one-half-feathered, flightless dinosaur called Epidexipteryx, which may exist an early pace on the road to birds, lives in China.

150 1000000 years ago

Archaeopteryx, the famous "commencement bird", lives in Europe.

140 one thousand thousand years ago

Around this time, placental mammals split from their cousins the marsupials. These mammals, like the modern kangaroo, that give birth when their young are all the same very small, just nourish them in a pouch for the starting time few weeks or months of their lives.

The majority of modern marsupials live in Commonwealth of australia, but they reach it past an extremely roundabout route. Arising in due south-east Asia, they spread into north America (which was attached to Asia at the time), then to south America and Antarctica, before making the final journeying to Commonwealth of australia about l million years ago.

131 million years ago

Eoconfuciusornis, a bird rather more advanced than Archaeopteryx, lives in China.

130 1000000 years ago

The kickoff flowering plants emerge, post-obit a catamenia of rapid evolution.

105-85 million years ago

The placental mammals split into their four major groups: the laurasiatheres (a hugely various group including all the hoofed mammals, whales, bats, and dogs), euarchontoglires (primates, rodents and others), Xenarthra (including anteaters and armadillos) and afrotheres (elephants, aardvarks and others). Quite how these splits occurred is unclear at present.

100 one thousand thousand years ago

The Cretaceous dinosaurs reach their peak in size. The giant sauropod Argentinosaurus, believed to be the largest land animal in Earth'due south history, lives around this time.

93 million years ago

The oceans become starved of oxygen, mayhap due to a huge underwater volcanic eruption. Twenty-seven per cent of marine invertebrates are wiped out.

75 1000000 years ago

The ancestors of modernistic primates split from the ancestors of modern rodents and lagomorphs (rabbits, hares and pikas). The rodents go on to be astonishingly successful, somewhen making upward around twoscore per cent of modern mammal species.

70 million years ago

Grasses evolve – though it will be several 1000000 years before the vast open grasslands announced.

65 million years ago

The Cretaceous-Third (Chiliad/T) extinction wipes out a swathe of species, including all the giant reptiles: the dinosaurs, pterosaurs, ichthyosaurs and plesiosaurs. The ammonites are also wiped out. The extinction clears the manner for the mammals, which go on to dominate the planet.

63 meg years ago

The primates divide into two groups, known as the haplorrhines (dry out-nosed primates) and the strepsirrhines (wet-nosed primates). The strepsirrhines eventually become the modern lemurs and aye-ayes, while the haplorrhines develop into monkeys and apes – and humans.

58 million years ago

The tarsier, a primate with enormous eyes to help information technology see at night, splits from the rest of the haplorrhines: the first to do and so.

55 million years ago

The Palaeocene/Eocene extinction. A sudden ascent in greenhouse gases sends temperatures soaring and transforms the planet, wiping out many species in the depths of the sea – though sparing species in shallow seas and on country.

50 one thousand thousand years ago

Artiodactyls, which look like a cross betwixt a wolf and a tapir, begin evolving into whales.

48 million years agone

Indohyus, another possible antecedent of whales and dolphins, lives in Bharat.

47 million years agone

The famous fossilised primate known every bit "Ida" lives in northern Europe. Early whales chosen protocetids live in shallow seas, returning to state to give nascency.

twoscore million years ago

New World monkeys become the first simians (higher primates) to diverge from the residuum of the grouping, colonising South America.

25 million years ago

Apes split from the Old World monkeys.

18 1000000 years agone

Gibbons go the first ape to split from the others.

14 million years ago

Orang-utans co-operative off from the other great apes, spreading across southern Asia while their cousins remain in Africa.

seven million years agone

Gorillas co-operative off from the other corking apes.

6 million years ago

Humans diverge from their closest relatives; the chimpanzees and bonobos.

Shortly afterwards, hominins begin walking on two legs. See our interactive timeline of human evolution for the full story of how modern humans developed.

2 one thousand thousand years ago

A 700-kilogram rodent called Josephoartigasia monesi lives in South America. It is the largest rodent known to have lived, displacing the previous tape holder: a behemothic guinea pig.

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