Our ecosystems are structured by feeding relationships like killer whales eating seals, which eat squid, which feed on krill. These and other animals require oxygen to extract energy from their food. With an environment devoid of oxygen and high in methane, for much of its history Earth would not have been a welcoming place for animals. The earliest life forms we know of were microscopic organisms microbes that left signals of their presence in rocks about 3.
The signals consisted of a type of carbon molecule that is produced by living things. Stromatolites are created as sticky mats of microbes trap and bind sediments into layers. Minerals precipitate inside the layers, creating durable structures even as the microbes die off. When cyanobacteria evolved at least 2. This catalyzed a sudden, dramatic rise in oxygen, making the environment less hospitable for other microbes that could not tolerate oxygen. Evidence for this Great Oxidation Event is recorded in changes in seafloor rocks.
When oxygen is around, iron reacts chemically with it it gets oxidized and gets removed from the system. Rocks dating to before the event are striped with bands of iron. Rocks dating to after the event do not have iron bands, showing that oxygen was now in the picture. After the initial pulse of oxygen, it stabilized at lower levels where it would remain for a couple billion years more. In fact, as cyanobacteria died and drifted down through the water, the decomposition of their bodies probably reduced oxygen levels.
So, the ocean was still not a suitable environment for most lifeforms that need ample oxygen. However, other innovations were occurring. While they can process lots of chemicals, microbes did not have the specialized cells that are needed for complex bodies. Animal bodies have various cells —skin, blood, bone — which contain organelles, each doing a distinct job.
Microbes are just single cells with no organelles and no nuclei to package their DNA. 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. Still, the fact that suggestive evidence of life arises right as the rock record begins raises a question, said University of California, Los Angeles, geochemist Elizabeth Bell in a SETI Talk in February : Is the timing a coincidence, or were there earlier forms of life whose remnants disappeared with the planet's most ancient rocks?
The period that occurred before the rock record begins is known as the Hadean. It was an extreme time, when asteroids and meteorites pummeled the planet. Bell and her colleagues said they might have evidence that life arose during this very unpleasant time. In , the research team reported discovering graphite, a form of carbon , in 4.
The ratio of isotopes in the graphite suggested a biological origin, Bell and her colleagues wrote in the journal Proceedings of the National Academy of Sciences. Meteorites or chemical processes might have caused the odd carbon ratios, she said, so the isotopes alone aren't proof of life. Since the publication of the paper, Bell said, the researchers have found several more of the rare-carbon inclusions, which the scientists hope to analyze soon. It is distinctly possible that this date will change as more evidence comes to light.
At some point far back in time, a common ancestor gave rise to two main groups of life : bacteria and archaea. How this happened , when, and in what order the different groups split , is still uncertain.
The oldest fossils of single-celled organisms date from this time. Some single-celled organisms may be feeding on methane by this time. Rock formations in Western Australia, that some researchers claim are fossilised microbes , date from this period. Viruses are present by this time , but they may be as old as life itself. Supposedly, the poisonous waste produced by photosynthetic cyanobacteria — oxygen — starts to build up in the atmosphere.
Recently, though, some researchers have challenged this idea. They think cyanobacteria only evolved later, and that other bacteria oxidised the iron in the absence of oxygen. Yet others think that cyanobacteria began pumping out oxygen as early as 2. Methane reacts with oxygen, removing it from the atmosphere, so fewer methane-belching bacteria would allow oxygen to build up. When the ice eventually melts, it indirectly leads to more oxygen being released into the atmosphere.
First 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. There is some evidence for an earlier date for the beginning of photosynthesis, but it has been called into question. One key organelle is the nucleus: the control centre of the cell, in which the genes are stored in the form of DNA.
The engulfed bacteria eventually become 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 relationship with them. The engulfed bacteria evolved into chloroplasts: the organelles that give green plants their colour and allow them to extract 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.
The eukaryotes divide into three groups: the ancestors of modern plants, fungi and animals split into separate lineages , and evolve separately. We do not know in what order the three groups broke with each other. At this time they were probably all still single-celled organisms. The first 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 stage similar to that of modern choanoflagellates : single-celled creatures that sometimes form colonies consisting of many individuals. Of all the single-celled organisms known to exist, choanoflagellates are the most closely related to multicellular animals, lending support to this theory.
The early multicellular animals undergo their first splits. First they divide into, essentially, the sponges and everything else — the latter being more formally known as the Eumetazoa. Around 20 million years later, a small group called the placozoa breaks away from the rest of the Eumetazoa.
Placozoa are thin plate-like creatures about 1 millimetre across, and consist of only three layers of cells. It has been suggested that they may actually be the last common ancestor of all the animals. The comb jellies ctenophores split from the other multicellular animals. Like the cnidarians that will soon follow, they rely on water flowing through their body cavities to acquire oxygen and food. 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.
Around 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. Little is known about how this happened. However, small worms called Acoela may be the closest surviving relatives of the first ever bilateral animal.
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