The tetrapods go on to conquer the land , and give rise to all amphibians, reptiles, birds and mammals. The oldest fossilised tree dates from this period. Tiktaalik , an intermediate between fish and four-legged land animals, lives around this time. The fleshy fins of its lungfish ancestors are evolving into limbs. The first major split occurs in the tetrapods, with the amphibians branching off from the others.
Within the remaining tetrapods, the sauropsids and synapsids split from one another. The sauropsids include all the modern reptiles, plus the dinosaurs and birds. The first synapsids are also reptiles, but have distinctive jaws.
The pelycosaurs, the first major group of synapsid animals, dominate the land. Despite appearances, Dimetrodon is not a dinosaur. The therapsids, close cousins of the pelycosaurs, evolve alongside them and eventually replace them. The therapsids survive until the early Cretaceous, million years ago.
Well before that, a group of them called the cynodonts develops dog-like teeth and eventually evolves into the first mammals. As the ecosystem recovers, it undergoes a fundamental shift. Whereas before the synapsids first the pelycosaurs, then the therapsids dominated, the sauropsids now take over — most famously, in the form of dinosaurs.
The ancestors of mammals survive as small, 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 great marine reptiles of the dinosaur era. Bird-like footprints and a badly-preserved fossil called Protoavis suggest that some early dinosaurs are already evolving into birds at this time. This claim remains controversial. As the Triassic period comes to an end, another mass extinction strikes, paving the way for the dinosaurs to take over from their sauropsid cousins.
Around the same time, proto-mammals evolve warm-bloodedness — the ability to maintain their internal temperature, regardless of the external conditions.
The first split occurs in the early mammal population. The monotremes, a group of mammals that lay eggs rather than giving birth to live young, break apart from the others. Few monotremes survive today: they include the duck-billed platypus and the echidnas. A half-feathered, flightless dinosaur called Epidexipteryx , which may be an early step on the road to birds, lives in China.
Around this time, placental mammals split from their cousins the marsupials. These mammals, like the modern kangaroo, that give birth when their young are still very small, but nourish them in a pouch for the first few weeks or months of their lives.
The majority of modern marsupials live in Australia, but they reach it by an extremely roundabout route. Arising in 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 journey to Australia about 50 million years ago. Eoconfuciusornis , a bird rather more advanced than Archaeopteryx , lives in China. The first flowering plants emerge, following a period of rapid evolution.
The placental mammals split into their four major groups: the laurasiatheres a hugely diverse 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. The Cretaceous dinosaurs reach their peak in size. The oceans become starved of oxygen , possibly due to a huge underwater volcanic eruption.
Twenty-seven per cent of marine invertebrates are wiped out. The ancestors of modern primates split from the ancestors of modern rodents and lagomorphs rabbits, hares and pikas.
The rodents go on to be astonishingly successful, eventually making up around 40 per cent of modern mammal species. Grasses evolve — though it will be several million years before the vast open grasslands appear.
The ammonites are also wiped out. The extinction clears the way for the mammals, which go on to dominate the planet. The primates split into two groups, known as the haplorrhines dry-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.
The tarsier, a primate with enormous eyes to help it see at night, splits from the rest of the haplorrhines: the first to do so. A sudden rise 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 land. Artiodactyls, which look like a cross between a wolf and a tapir, begin evolving into whales.
Indohyus , another possible ancestor of whales and dolphins , lives in India. Early whales called protocetids live in shallow seas, returning to land to give birth. New World monkeys become the first simians higher primates to diverge from the rest of the group, colonising South America. Apes split from the Old World monkeys. Gibbons become the first ape to split from the others. Some genes act like orchestra conductors, controlling the expression of many other genes at specific places and times to correctly assemble the components.
While they were not played out immediately, there is evidence that parts of instructions for complex bodies were present even in the earliest animals. Thanks to their hard skeletons, sponges became the first reef builders on Earth.
Klaus Ruetzler are working to understand the evolution of the thousands of sponge species living on Earth today. By about million years ago the Ediacaran Period there was a proliferation of other organisms, in addition to sponges.
These varied seafloor creatures - with bodies shaped like fronds, ribbons, and even quilts - lived alongside sponges for 80 million years.
Their fossil evidence can be found in sedimentary rocks around the world. However, the body plans of most Ediacaran animals did not look like modern groups.
Douglas Erwin , using comparative developmental evidence, has examined whether any of the fossilized Ediacaran animals were related to modern animals. By the end of the Ediacaran, oxygen levels rose, approaching levels sufficient to sustain oxygen-based life. The early sponges may actually have helped boost oxygen by eating bacteria, removing them from the decomposition process. Tracks of an organism named Dickinsonia costata suggest that it may have been moved along the sea bottom, presumably feasting on mats of microbes.
However, about million years ago, most of the Ediacaran creatures disappeared, signaling a major environmental change that Douglas Erwin and other scientists are still working to understand. Evolving animal body plans, feeding relationships, and environmental engineering may have played a role.
Burrows found in the fossil record, dating to the end of the Ediacaran, reveal that worm-like animals had begun to excavate the ocean bottom. These early environmental engineers disturbed and maybe aerated the sediment, disrupting conditions for other Ediacaran animals. As environmental conditions deteriorated for some animals, they improved for others, potentially catalyzing a change-over in species.
The Cambrian Period million years ago witnessed a wild explosion of new life forms. Along with new burrowing lifestyles came hard body parts like shells and spines. Hard body parts allowed animals to more drastically engineer their environments, such as digging burrows. A shift also occurred towards more active animals, with defined heads and tails for directional movement to chase prey. Active feeding by well-armored animals like trilobites may have further disrupted the sea floor that the soft Ediacaran creatures had lived on.
Unique feeding styles partitioned the environment, making room for more diversification of life. While Waptia scoured the ocean bottom, priapulid worms burrowed into the sediment, Wiwaxia attached to sponges, and Anomalocaris cruised above. Many of these odd-looking organisms were evolutionary experiments, such as the 5-eyed Opabinia. However, some groups, such as the trilobites, thrived and dominated Earth for hundreds of millions of years but eventually went extinct.
Stromatolite reef-building bacteria also declined, and reefs made by organisms called brachiopods arose as conditions on Earth continued to change. This characteristic is distinctive of bacteria and archaea; all other life forms on Earth, including real algae, consist of eukaryotic cells with organelles and with genetic material contained in one place the nucleus.
Bacteria and archaea are hardy creatures. They thrive in hot, cold, salty, acidic and alkaline environments in which most eukaryotes would perish. Despite this, they have a bad image: after all, bacteria cause many diseases in humans. Yet without them we may not be here at all. Cyanobacteria then went a step further: they started to utilise water during photosynthesis, releasing oxygen as a by-product. But we may owe bacteria more than the air we breathe.
It is likely that eukaryotic cells, of which humans are made, evolved from bacteria about two billion years ago. One theory is that eukaryotic cells evolved via a symbiotic relationship between two independent prokaryotic bacteria.
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