Most people accept that all life is subject to evolution. Charles Darwin summed it up in his book titled “The Origin of Species”. But, what is the real origin of species? By this, I do not mean the common ancestor we shared with apes. To date, we have still not found any remains of this common ancestor. What I am referring to in this article is the last common ancestor of all types of life.
This is termed the Last Universal Common Ancestor or LUCA. It is important to point out that we are not looking for any ancient creature. We need to go much further back from the first animals. What we are looking for is the common ancestor of what are called the first three “domains of life.” These three domains are Bacteria, Eukarya and Archaea. So, our target is something very small.
We have not found it but scientists speculate it may have the following characteristics. By checking the modern gene codes of around 2,000 microbes of bacteria and archaea, they ended up with an initial total of 6 million genes from which they found 355 gene families which could be traced back. Therefore, the LUCA could take the form of a “cellular organism, having a lipid bilayer”. It is thought it would have “used a protein as well as DNA and RNA”. How far back in time should we be looking for this organism? The Earth was formed around four and a half billion years ago. Initially, conditions would have been such as to make any form of life as we know it impossible to survive. Therefore, we are looking at the region of 3.5 to 3.8 billion years ago.
Moving forward in time, what were the earliest forms of life on Earth? As we have no fossil evidence, as highlighted above, we have to make a deduction based on the genetic codes available to us today and attempt to trace these back to the beginning of life on Earth. The conclusion reached is that these early lifeforms would have lived in the oceans which would have been of a high temperature.
This would likely have been due to all the ocean bottom vents which were spewing out hot magma. Remember, at this stage of the planet, it was extremely active. We know these hydrothermal vents existed because there are still some active at the bottom of the oceans. Although you may think that living next to one of these would kill every form of life, this is not so. We have found life living happily around them. These vents issue dangerous sulphides and metals. Our LUCA did not need oxygen to breathe. Instead, it would probably exist on hydrogen.
There is another theory that life began on land and then made its way down to the hydrothermal vents. It first existed in pools on the surface being aided by the Sun’s ultraviolet rays. But, around 4 billion years ago the Earth was bombarded by asteroids and comets. LUCA may have then taken refuge in the oceans.
The truth of the matter is that we still do not know for sure, but research is getting closer to finding the root of our Tree of Life.
John Pullen 