Friday, 19 September 2008

What Was Early Earth Like?

In the scientific community there are two main schools of thought on the origin of life on Earth. One is that life was started here via a process known as panspermia. This is where bacteria are carried through space from another planet on a comet or meteor. The idea is that these bacteria would somehow survive impact and then go on to flourish on the new planet, in this case Earth.

While the theory of panspermia is certainly valid, Earth was heavily bombarded by meteorites prior to the start of life, it still doesn't answer how life came to be in the first place. The likelihood of something surviving an impact combined with landing somewhere where it could thrive seems like a remotely small chance as well. The more widely accepted theory is that life on Earth originated here and did not come from another planet.

No concrete theory has been put forward that shows how life can come into being from non-living material yet. Without perfectly sterile lab conditions and a few hundred million years it may prove impossible. However it is theorised that life began as a series of organic chemicals and molecules. One of these molecules gained the ability to replicate itself. The crucial feature of this replication is that it was not perfect, there would sometimes be 'errors' in the reproduction.

If an 'error' in reproduction left that particular strand at a disadvantage then they would not reproduce as much and would eventually die out. If an error or mutation left a strand at a clear advantage, however, then they would reproduce in greater numbers and would take over. This is the start of natural selection and evolution, the theory that is thought to govern the development of all life right from the beginning to where we are today. Eventually DNA would emerge as the dominant force in replication and life was well on its way to becoming cellular.

Eventually primitive life would evolve membranes and so cells were born. The leap to membranes is actually easier to comprehend as the phospholipids present in cell membranes form bilayers spontaneously when placed in water. This property would allow the contents of life to be contained within the confines of a cell. The cells that would go on to become the ancestors of all life on Earth were present at this time, just over 3.5 billion years ago. They were primitive compared to modern cells, still lacking a nucleus, mitichondria and chloroplasts.

The next main development that would change the planet came with the advent of photosynthesis. Photosynthesis would allow life to use sunlight as an energy source. Cells that did not develop photosynthesis would also benefit as they could consume the photosynthesising cells. The secondary effect of stimulating the growth of life was what is know as the 'Oxygen Catastrophe'. Oxygen is a waste product of photosynthesis and was toxic to a great many organisms. The waste oxygen would have first become bound to limestone and iron in the sea, before it could escape into the atmosphere. The atmosphere would slowly build up in concentration of oxygen and the ozone layer eventually formed, guarding the planet 's surface from harmful ultraviolet radiation. As a substantial amount of life died from oxygen some would find a way to harness it to enhance their metabolism.

Cells would continue to adapt over millions of years, gaining organelles and nuclei. At this point in history, around 1.1 billion years ago, the first supercontinent was beginning to take shape and plant, animal and fungus cells had split and were clearly defined, although still single-celled. As colonies of cells built up in volume over time some cells in certain parts of the colony would evolve into specialist cells giving a benefit for the whole colony. As this division began to take place it could be said that this was the start of multicellular organisms. Around 900 million years ago the first multi-celled animals began to appear.

Around 770 million years ago scientists believe that the planet went through a phase that has become known as Snowball Earth. For 20 million years the oceans froze over and the Earth entered a severe ice age. Eventually after time volcanic eruptions would cause enough carbon dioxide to enter the atmosphere and heat the planet due to the greenhouse effect.

By 530 million years ago fish had evolved and were the first known vertebrates. With the ice melted, the ozone layer present and complex life thriving in the ocean it would only be a matter of time before life would take to land.

1 comment:

  1. When I was in of the theories we've studied is that life forms on Earth all started from a primordial soup that contains all the basic elements that can sustain life...however, this theory comes into conflict with what religion teaches us about how life started..anyways, whichever theory is true what's important is that we are here and we are doing everything to preserve our gift of life.