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Get your Darwin Tree of Life Poster Free
and Evolution by Chris Pamplin
Life through Time by Pyers Symon
Trying to get your head around the history of life on this planet is difficult because of the unimaginable time spans involved! A lot of the early history is understandably vague and although scientists have ideas about what happened way back then, trying to be precise is always going to be tricky if not impossible ..
The earth is estimated to be about 4.5 billion years ago; 4,500,000 years. Imagine a human life span (nowadays anyway) to be about 80 years. Now imagine these people to have their lives run one after another there would be an astonishing 56million life spans involved.
So when did life begin? It started pretty soon - relatively speaking of course ! - after the formation of the earth. How life evolved abiogenesis is still a bit of a mystery with many issues to be sorted out but a basic idea is that some molecules managed to find methods of replicating themselves (how that happened exactly nobody knows) but it happened about 4 billon years ago - only a remarkable few hundred million of years following the earth's formation.
Once basic biochemistry
had started, natural selection would have kicked in, promoting those replicators
that were best "fitted" to their environment.
So when did the first cells appear? Well .. sometime around 3.5 billion years ago the earliest cells appeared . These are known as prokaryotes and resemble modern bacteria and a group, which came a bit later, called the archaea.
Eon 3.8-2.5 billion years ago
This oxygen was initially stored in the oceans but eventually started to change the atmosphere which became much more like it is today - 20 % O2 . The problem was that oxygen is very toxic to a lot of organisms (still is !) so new organisms evolved which could use oxygen - these aerobic organisms which could use oxygen were vastly more efficient energy wise than their anaerobic predecessors (in fact anything up to 19 times more efficient!) and caused a major explosion in life.
The first fossils came from around them - large mats of fossilized bacteria called cyanobacteria can be found. These are called stromatolites and are pretty common in some parts of the world - the earliest being from about 2.7 billion years ago
Proterozoic Eon 2500 billion - 542 million years ago (MYA)
Sex was invented about then which allowed the genetic information to be mixed and advantageous genes to be spread more efficiently, although the origins of sexual reproduction is still a bit of a mystery!
Also about then, simple multi-cellular organisms appeared - cell colonies. About 580 mya the first proper multi-cellular organisms appeared called the Ediacara biota.
the most extraordinary events in the earth's history happened around 580 mya -
Explosion. Around then something triggered a massive increase which caused
pretty well all of the modern groups of modern animals and plants to appear
Also the amount of oxygen in the atmosphere increased to such an extent that an ozone layer formed . This would block UV and allow life to move onto the land
Phanerozoic Eon 542 MYA- NOW
Palaeozoic Era 542-251 MYA, Ancant Life
Consists of the Cambrian,Ordovician,
Over the eons there have been a number of events when the number of species on earth crashed (we will be meeting a very famous one later!) and one happened about 250 MYA. This, in fact, was the biggest mass extinction ever with about 95% of all marine species disappearing (including the trilobites) and about 70% of land vertebrates getting the chop This is technically called the "Permian-Triassic (P-T) extinction event" or informally "The Great Dying" It was the closest that life on earth came to vanishing.
Following the P-T extinction events, many new life forms appeared to fill the void: Turtles started swimming in the sea and ichthyosaurs (there is a cracker in Charmouth) were there with plesiosaurs. Ammonites appeared and define the start of the Jurassic period.
The Jurassic period has the seas full of fish; ammonites and belemnites. Ichthyosaurs continued to swim, but there were marine crocodiles as well In the Cretaceous huge beds of chalk were laid down from the shells of tiny algae called coccoliths .
Later the Cretaceous was dominated by the famous dinosaurs - the terrible lizards. Mammals began to appear as did the birds. The first flowering plants were to be found
A meteorite slammed into what is now the Gulf of Mexico .This is known as the K-T extinction event (or more modern the Cretaceous-Paleogene event)
Huge numbers of species vanished - the dinosaurs (including our friend T Rex) , ammonites (up to about 40% of all marine species became extinct), but a lot of mammals survived (they were much smaller when compared with dinosaurs) and hence were able to keep going.
evolved into what we now have. Birds
(which had first appeared in the late Mesozoic) became abundant, flowering plants
became, with grasses, the dominant flora.
The primates appeared and about 5 million years what were to become humans split from the line which became chimpanzees.
Modern Humans started to walk the earth about 250,000 years ago
In other words humans have been on this planet for only about 0.00625% of the time that life has been on this planet ..
Further reading and DVD
I would strongly recommend Richard Dawkin's superb "The Ancestor's Tale" where he steps us back through time and biological space to the origin of life via a series of what he called "concestors" - common ancestors (a slightly ugly word I am afraid) , which are splitting points where species divided. For instance, the first common ancestor of humans (with chimpanzees) happened about 6 million year ago and then Dawkins takes us back through a further 38 steps - making a Buchanesque 39 Steps - back to the origin of life itself . Get the hardback edition !
an outline of current(ish) theories concerning the origin of life have a look
Our Closest Relatives in the Animal Kingdom by Chris Pamplin
Humans, Homo sapiens are just one of the group of Mammals known as the order Primates. We have evolved through many ancestral stages. About 30,000 years ago there lived two different species of Human, Homo neanderthalensis or Neanderthal Man and Homo sapiens. Neanderthal humans were adapt at living in the cold ice age of Europe but soon migrating tribes of Homo sapiens ( us) would move from Africa encroaching on their lands. Whilst it is conjecture, it is not unbelievable to assume that one species wiped out the other. Some Scientists think the two species interbred, there may even be a little Neanderthal in all of us!
Our closest relatives are now the Chimpanzee with whom we share some 95% of our DNA and the Bonobo. Humans are not the top of the evolutionary tree but a branch of the tree which is still evolving. We have come to dominate and change our entire planet and have even flown into space ( mind you the first Hominid into space was a Chimpanzee in 1961, sent there by us). We are in a position to effect all life on Earth and while we are capable of tremendous good we are also a savage species which fights, goes to war and destroys it's own kind as well as other species.