24 February, 2008
Looking back to the past, scientists have narrowed down the age of the universe, the age of the earth and how long ago life started. Current estimates put the beginning of the universe at about 13.7 billion years ago. The Earth formed along with the rest of the solar system about 4.6 billion years ago. There is evidence that there was life on Earth about 3.8 billion years ago, and there are fossils from 3.5 billion years ago which are visible to the naked eye. These fossils are stromatolites, large mats of bacteria. At this time there was no multicellular life, but these large groups of bacteria stuck close together as they multiplied and formed mounds that were up to 30cm wide. They lived in the shallow seas, gathering energy from the sun. At this time there was no thick ozone layer to protect life from ultraviolet radiation and so life tended to stick to the oceans. Since the sun was a good source of energy a lot of life hugged the shallow coasts of the continents where the benefits of the sun could be had while the water protected the vulnerable life from the more harmful radiation.
Life at this time was not as complex as it is now. The cell is rightly seen as a marvel of chemistry, a miniature factory that cannot have evolved in any small amount of time. Indeed it did not evolve overnight, and though it is unclear exactly when very complex cells and multicellular life first arose, it is not until the Vendian Period, 620 million years ago, that there is any evidence of multicellular life in the fossil record. So from the beginning of life, it took over 3 billion years before there were complex creatures that could perhaps be called animals. Even here the animals were more like jellyfish and worms, with no hard parts like shells or bones. They finds of fossils from this period indicate that they were made of harder material than jellyfish today, but even so the seas of this time would have no fish, no squid, no swimming creatures that hunt or hide. The land above would perhaps have some bacteria on it, but nothing would walk the surface and nothing flew in the air. No insects or birds would disturb a traveller to this time.
Once multicellular life had gotten a hold it wasn’t long before it developed some amazing specialisation. By being able to have specific cells for specific functions, and also by introducing sex for the first time, evolution and natural selection took off. The Cambrian explosion is the term used to describe the appearance of a huge amount of new species in the fossil record about 540 million years ago. Many of the body forms of major groups of creatures today can be seen in their ancient form in the fossils from this time. Also by this time the oxygen content of the atmosphere had risen from practically nothing 3 billion years before to something near it’s present level. This oxygen interacted with solar radiation to create ozone which allowed creatures to live out of the oceans for the first time. And here on land life began to thrive.
It took the vast majority of the time that the Earth has existed to get to this stage. In those 3 billion years the groundwork had been laid that would allow the colonisation of almost every part of the planet by life. If you were to randomly time travel to some part of the Earth’s history, not only is it extremely unlikely that you would meet another human, it’s also quite unlikely you would meet a mammal or even any animal at all. We multicellular beings are fairly recent additions to the planet but through our use of complex bodies, and the power of sex in creating new variation, we have come a long way in a mere 500 million years.