It seems likely that the first bilateral animal was a kind of worm. However, small worms called Acoela may be the closest surviving relatives of the first ever bilateral animal. 630 million years agoĪround this time, some animals evolve bilateral symmetry for the first time: that is, they now have a defined top and bottom, as well as a front and back. The ancestor of cnidarians (jellyfish and their relatives) breaks away from the other animals – though there is as yet no fossil evidence of what it looks like. Like the cnidarians that will soon follow, they rely on water flowing through their body cavities to acquire oxygen and food. The comb jellies (ctenophores) split from the other multicellular animals. The planet freezes over again in another “ snowball Earth“. It has been suggested that they may actually be the last common ancestor of all the animals. Placozoa are thin plate-like creatures about 1 millimetre across, and consist of only three layers of cells. First they divide into, essentially, the sponges and everything else – the latter being more formally known as the Eumetazoa.Īround 20 million years later, a small group called the placozoa breaks away from the rest of the Eumetazoa. The early multicellular animals undergo their first splits. Of all the single-celled organisms known to exist, choanoflagellates are the most closely related to multicellular animals, lending support to this theory. It is unclear exactly how or why this happens, but one possibility is that single-celled organisms go through a stage similar to that of modern choanoflagellates: single-celled creatures that sometimes form colonies consisting of many individuals. The first multicellular life develops around this time. At this time they were probably all still single-celled organisms. We do not know in what order the three groups broke with each other. The eukaryotes divide into three groups: the ancestors of modern plants, fungi and animals split into separate lineages, and evolve separately. The engulfed bacteria evolved into chloroplasts: the organelles that give green plants their colour and allow them to extract energy from sunlight.ĭifferent 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. Later, eukaryotic cells engulfed photosynthetic bacteria and formed a symbiotic relationship with them. The last common ancestor of all eukaryotic cells had mitochondria – and had also developed sexual reproduction. The engulfed bacteria eventually become mitochondria, which provide eukaryotic cells with energy. One key organelle is the nucleus: the control centre of the cell, in which the genes are stored in the form of DNA.Įukaryotic cells evolved when one simple cell engulfed another, and the two lived together, more or less amicably – an example of “endosymbiosis”. 2 billion years ago?Įukaryotic cells – cells with internal “organs” (known as organelles) – come into being. There is some evidence for an earlier date for the beginning of photosynthesis, but it has been called into question. 2.15 billion years agoįirst undisputed fossil evidence of cyanobacteria, and of photosynthesis: the ability to take in sunlight and carbon dioxide, and obtain energy, releasing oxygen as a by-product. When the ice eventually melts, it indirectly leads to more oxygen being released into the atmosphere.
Science Photo Library/Getty Images 2.3 billion years agoĮarth freezes over in what may have been the first “snowball Earth”, possibly as a result of a lack of volcanic activity. Methane reacts with oxygen, removing it from the atmosphere, so fewer methane-belching bacteria would allow oxygen to build up. Yet others think that cyanobacteria began pumping out oxygen as early as 2.1 billion years ago, but that oxygen began to accumulate only due to some other factor, possibly a decline in methane-producing bacteria. They think cyanobacteria only evolved later, and that other bacteria oxidised the iron in the absence of oxygen. Recently, though, some researchers have challenged this idea. Dissolved oxygen makes the iron in the oceans “rust” and sink to the seafloor, forming striking banded iron formations. Supposedly, the poisonous waste produced by photosynthetic cyanobacteria – oxygen – starts to build up in the atmosphere. Viruses are present by this time, but they may be as old as life itself. Rock formations in Western Australia, that some researchers claim are fossilised microbes, date from this period. Some single-celled organisms may be feeding on methane by this time. The oldest fossils of single-celled organisms date from this time. How this happened, when, and in what order the different groups split, is still uncertain.
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