Timeline of Earth
Note: No date from prehistoric Earth can be completely definite. I have done my best to choose reliable dates, but there is much contention with conflicting evidence. Hopefully these diagrams will give a sense of the changes and events occurring within our planet’s history, but I cannot guarantee complete and long lasting accuracy.
The above image is a timeline of Earth’s history, from its formation 4,540 million years ago (mya) to now. The first Eon, Hadean, was a tumultuous one, the planet was still very hot and volcanic and the air was mostly made of carbon dioxide. Although oceans formed about 4,300 MYA ago, they were vapourised by a meteor bombardment that lasted until the Archean Eon.
The Archean Eon saw the first microscopic life, with Bacteria and Archaea evolving, plus the process of photosynthesis started to occur. The Oxygen Catastrophe, a massive increase in oxygen that killed most of the planet’s anaerobic bacteria, occurred in the Proterozoic Eon and was caused by photosynthesizing Cyanobacteria. This also led to the longest period of glaciation (shown by the white edges of the timeline). Acritarch and Grypania are fossils from this time and are thought to be evidence of the first Eukaryotic cells (it is not definite whether either or both are Eukaryots, so I have included both). The multicellular Bangiomorpha is the the first organism to reproduce sexually, and this was a very important development. Without sexual reproduction there could be no mixing of DNA from two parents, which meant that natural selection and consequently evolution, were extremely slow. After Bangiomorpha the change from multicelled, but very simple organisms, to the wide and complex variety of living things we have today, is comparatively fast.
800mya the first multicelled organisms evolved, algae – Cladophora, and around this time another glaciation occurred. The first fungi arrived 700mya, the first animals 665mya. All these organisms lived only in the sea.
The Cambrian Explosion, 530mya, was when many animals appeared, they were arthropods and many of these were crustaceans (see previous blog for more information), here is a great animation of some of those animals. The first big extinction event happened about 50 million years later, in the Phanerozoic Eon (shown on the diagram with a vertical line of crosses), and led to the loss of many animals, especially Trilobites, one of the first Arthropods to have existed. It is interesting to note that while there have been a number of animal extinctions (six can be seen on the diagram) there have been no known plant extinctions and I could find no record of fungi extinctions. This is probably because both plants and fungi are able to survive extreme trauma, regenerating from small portions of root or mycelium. Animals, however, tend die quite easily, the more sophisticated the animal, the more vulnerable it tends to be. Hit an animal on the head or set fire to it and it will probably be dead, but do the same to a plant or fungus and, given a bit of time, they will flourish again.
The Phanerozoic Eon, 542-0mya (we are still in this Eon today) also saw the move onto land by animals, algae and fungi. Whereas animals and fungi had evolved into sophisticated enough forms to be classified in the Fungi and Animal Kingdoms, plants hadn’t appeared at that stage, there was only algae from the Protista Kingdom (see previous blog). Once on land, plants evolved quickly in order to prevent dessication and make use of the greater access to sunlight; seashores played an important part in this transition due to the shifting back and forth from dry to wet. 200mya was the Age of Reptiles, a time of dinosaurs, and their extinction was followed by the Age of Mammals.
- All pictures of plants are symbolic representations only, for example the first orchids were unlikely to have looked like the Phalaenopsis that I have drawn.
- A slightly more accurate, but less clear version of this diagram, with smaller pictures of the plants can be seen by clicking on this thumbnail. The picture of the plant, rather than the writing, marks the time when it is believed to have first appeared.
Plant evolution happened, like most evolution, in spurts. The first spurt happened during the Silurian and Devonian Period (Periods are subdivisions of Eons, or to be more accurate, Eons are subdivided into Eras which are further subdivided into Periods which are further divided into Ages or Stages, but not until the Phanerozoic Eon. See thumbnail at end of blog.) and involved two important changes, first, soils were created and second, proto-plants developed a cuticle, a waxy covering that stopped them drying out. These two alterations led to the first plants able to grow on land around 475mya, these were probably small non-vascular plants such as mosses or liverworts, sometimes described as bryophytes. Bryophytes do not have properly formed leaves, roots or stems and were, and are, still very reliant on water, this is why they can only be found growing in moist, often shady areas.
Shortly afterwards, vascular plants evolved, Cooksonia, a plant that no longer exists, was one of these and first appeared around 428mya. Plants in the Division Lycopodiophyta were the first vascular plants that still live now. The vascular system is made up of two tubes that travel to all parts of the plant, these are called the phloem and xylem. The xylem carries water from the roots to all parts of the plant and the phloem transports sugars that are made by photosynthesis in the leaves. This system enabled the plants to grow tall and in fact horsetails were part of the first forests (occurring around 350mya) and grew to 30m tall, nowadays they rarely grow more than 150cm tall. Ferns, horsetails and bryophytes all reproduce by spores, a method that requires water.
The next leap forward was conifers in the Carboniferous Period (299-352mya), these had a more developed vascular system than the ferns, and could, and can, grow in drier areas. They also evolved pollen and seeds, rather than spores. Seeds were useful because they were tougher than spores, able to survive for long periods, and relied less on water. Gnetophytes and Ginkgos evolved in the Permian period (252-298mya). These three divisions (Cycadophyta, Gnetophyta and Ginkgophyta) do not contain many species still in existence. Cycadophyta has only three living families, one of which is Cycadaceae which contains palm-like plants (although palm trees are actually flowering plants and only evolved 90mya). Gnetophyta has only three genera and they are odd looking plants, such as Welwitschia mirabilis. Finally Ginkgo which now has only one species, Ginkgo biloba.
Flowering plants the Angiosperms, did not evolve until the Cretaceous Period about 140mya. In the following 100 million years there was another spurt, monocotyledons evolved and orchids (80mya), lilies (60mya) and grasses (50mya) followed, all of which are monocots. Cacti also evolved around this time, about 30mya.
Note: I haven’t shown humans on either timeline, this is because we are such a recent addition that there is not enough detail on the diagrams to be able to mark our appearance. Both timelines mark out every 50 million years, but the genus Homo only evolved 2 million years ago, and the species Homo sapiens 200,000 thousand years ago. Agriculture probably first occurred 12,000 years ago.
Note: On researching further I realised I had made a few errors in this blog, hopefully they are all now corrected.