Many imagine the early Earth as covered in molten rock. Fact is the Earth’s crust solidified within a hundred million year after the birth of the Moon. 200 million years later the Earth was almost completely covered by seas. Continents did not exist from start but formed a little by and by. The rock they consist of can be up to thousands of millions of years old. The ocean floors are almost entirely recycled within a couple of hundred million years. Parts of the
It is uncertain when life on Earth arose too. The oldest chemical traces of life come from the Isua formation on
When cyanobacteria had arisen they released large amount of oxygen. Iron and aluminium rich sediments at first absorbed it. This has been described as the whole Earth rusting. When such sediments were saturated oxygen stated to accumulate in both seas and air. This was 2,850 million years ago. Since few organisms then could handle oxygen this resulted in a mass extinction. However, at the same time this paved the way for oxygen-requiring organisms. Later eras and periods I have listed here.
There is great uncertainty on when the eukaryotes arose. In contrast it is rather certain how this process came about. Archaea resemble bacteria in their relatively simple cells. The difference is they have the ability to exchange genes with each other. At some point in the Paleoproterozoic one type enclosed its genes with an internal membrane. This became the first cell nucleus which characterizes all eukaryotes. After it symbiosis arose with an oxygen requiring bacterium. So the mitochondria came to be which nearly all eukaryotes have. (The few not having these have lost them later.) Long later symbiosis also arose with cyanobacteria. At three different occasions this has taken place. The results were photosynthetic eukaryotes usually called algae. However, they don’t have any common origin.
For long all life on Earth consisted of microbes. They lived either as microbial mats or as plankton. When multicellular organisms arose they lived adjacent to existing microbial mats. Fossils of photosynthetic eukaryotes have been found from the Statherian. To me it seems plausible fungi arose during the Tonian. Multicellular organisms of unclear classification arose during the Cryogenian. Some which can definitely be called animals can be traced back to the Ediacaran. All animals found from this time period ate organisms other than animals. Those who died from diseases or accidents were broken down directly by microbes.
Sponges and cnidarians arose during the Ediacaran. Fossil of animals with two sides has been found from this period too. The establishment of these groups had a large effect on the seas. Plankton eaters created lumps of waste which were harder to break down. At the same time worm-like animals ate up the microbial mats’ organisms. Waste accumulated on the bottom and microbial mats become rare. Both lead to decreased oxygen consumption and rising oxygen content in the seas. This made it possible for animals to evolve more complicated forms. Over the course of 25 million years most animal phyla arose. Examples of these are arthropods, chordates, echinoderms and molluscs. As part of this process predators arose as well as specialised scavengers.
Nearly all current phyla are found as fossils from the Cambrian. But the very most still lived adjacent to the sea floor. During the Ordovician animals started to spread more trough the water masses. In particular it was cephalopods which came to dominate the seas. The closest thing to fish to exist resembled present-day cyclostomes and lacked many specialised organs.
At the same time multicellular life started to establish itself on land. Relatives of present-day green algae gave rise to liverworts. Fungi adapted to living on land too. Some broke down dead moss. Other formed lichen with photosynthetic microbes. Fungi, lichen and liverworts increased the speed of erosion by breaking down rock. The result was drastic global cooling causing a mass extinction. I think it got as cold as the peak of the last three ice ages. The temperature sunk so quickly organisms did not have time to adopt. Furthermore, most lived right outside the coasts. Few coast stretched in the north-south direction making it hard to escape. The organisms coping best were those completely living freely in the water. This applied to many cephalopods and some jawless chordates. Both the Ordovician and the Silurian were for this reason dominated by cephalopods.
During the Silurian chordates with separate yaws evolved. This meant cartilaginous fish and later bony fish. At the same time arthropods went up on land. There they gave rise to arachnids and various myriapods. Towards the end of the period plants with roots evolved. However, they were very shallow so the vegetation was limited to moister climates. Neither were they particularly large. The largest organism on land I think was a gigantic lichen.
The Devonian has long be considered the age of fish. A large number of bigger plants evolved then too. Lycophytes grew as large as trees with oversized stalks for trunks. In the middle of the period real trees with wood arose. This was a now extinct group reproducing by spores. Towards the end of the period ferns arose. Before the Devonian was to an end seed ferns evolved. Although they resembled ferns they were the first to bear seeds. They took the help of the wind to reproduce and did not need moist ground.
All this vegetation increased erosion additionally. The climate for sure become colder but this trend was reduced by increased volcanism. Instead the main problem consisted of regional eutrophication. This benefited photosynthetic microbes which filled large areas of sea. The breaking down of all this biomass consumed too much oxygen. Nearly all life suffocated in the affected areas of sea. At least eight such disasters happened during the latter half of the Devonian. At the very end of the period a large star died too close to the Earth. The ozone layer was globally destroyed and exposed life on land to radiation. A great deal of organisms went extinct due to all the radiation damage. However, life in the sea suffered the worst from all the combined hypoxia.
During the Carboniferous the air’s oxygen content rose to tangibly higher than today. This made large animals possible even if their respiratory system were badly inefficient. The insects arose as the latest at the very start of the Carboniferous. Not only relatives of the dragonflies came to reach giant size. A type of millipede reached at least the length of a human. The largest scorpions become at least 70 centimetres (2 feet 4). Amphibians arose from fish which had evolved legs in shallow waters. Within a few tens of millions of years reptiles had evolved from them. Conifers also arose over the course of the Carboniferous.
In the early Permian the cycads arose. They strongly resemble palms despite not being flowering plants. In contrast to real palms they reproduce by cones. Reptiles with large fins on their backs also existed in the early Permian. To me it is more interesting which animals succeeded them. 280 million years ago relatives of these began to evolve warm-bloodedness. Higher metabolism means faster growth and faster reproduction rate. These in turn means a faster pace of evolution. In less than 10 million years proto-mammals took over. They then filled many niches having until then been empty. No animals had the potential to evolve to anything such before!
Please note the Earth’s average distance to the Sun has virtually not changed. The Milanković cycle for the Earth’s orbit is about exactly how elongated it is. Neither have size or gravity changed to any larger degree. What has primarily changed is how fast the Earth rotates. This speed has gradually decreased over the course of thousands of millions of years. The decrease has not been constant because it is affected by the geography and the global climate. The Earth and the Moon influence each other since they move at different speeds. The Moon orbits much slower than the Earth rotates. This makes the Earth spin slower and the Moon’s orbital time increase. But the Earth’s and the Moon’s movements will never synchronise. The Sun will die before that.
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