LUCA and Other Musings

 

LUCA

Recently I had a discussion with a young Jehovah’s Witness. The discussion soon turned to evolution and it became very clear to me that my young protagonist had no idea concerning the mechanism of evolution. No surprise I suppose as the JWs are only allowed to read texts about evolution that have been penned by the JW hierarchy and these texts exclusively emphasise how evolution is not a valid driver for biological change, thus paving the way for creationism. He had been taught that scientists believe that man had evolved directly from the great apes such as gorillas and chimpanzees. Actually, this is a misconception held by many lay folk and is not exclusive to JWs. This is not how evolution works and I explained that humans and great apes were derived from a common ancestor loping about some 6 million years ago. Humans and apes then diverged to pursue their own unique evolutionary histories and pathways culminating in modern humans and modern apes. He had never been taught or had contemplated this scenario before and appeared genuinely nonplussed.

Not only do we share a common ancestor with the Pongids, but by extension, to every other living creature on earth, even the humble Prokaryote bacterium. Prior to 1977, life could be classified as belonging to one of two domains: Prokaryotes and Eukaryotes. Prokaryotes refer to bacteria, Eukaryotes are more complex multicellular organisms and this broad domain includes creatures as diverse as fungi and humans. In 1977 the science of Taxonomy broadened to include a domain subsequently classified as Archaea. Archaea are single-celled organisms, vaguely similar to bacteria, but simpler. At some point in evolutionary history, the three main domains converged and consequently shared a common ancestor. Is this mere conjecture from biologists indulging in a drug filled reverie or is it evidence based? The genetic revolution over the past 40 years has allowed scientists to gain access to genetic evidence suggesting a common ancestor to all current life on Earth. A comparison of the genetic blueprint from diverse species enables geneticists to check for gene commonality. Genes have been discovered which appear in all species so far examined. It matters not that it be a bacterium, toadstool or human, these genes are always present. The implication, of course, is that these universal genes arose from a common ancestral organism or: Last Universal Common Ancestor (LUCA). Also, genetic technology enables researchers to reach back to uncover the genetic makeup of this mother of all life. A putative genomic reconstruction suggests that LUCA was sustained by 500 to 600 genes.  Furthermore, it is likely that the ancestral genome was RNA based. All living organisms extant are DNA based. DNA is a much better molecule for the conveyance of genetic instructions and for the faithful reproduction of information. However, DNA is a more complex molecule than RNA and the first life is more likely to have started with simpler molecules. The evolution from RNA to DNA based life forms would have represented one of the major milestones in the development of life.

LUCA represents a very primitive form of life and may have existed some 3.8 billion years- only 700,000 years after the formation of the earth. This is not to say that LUCA was the ‘first life’. It has been hypothesised that other life forms were present, but LUCA represents the organism which spawned all life we see today. Other competing life forms were ultimately unsuccessful and have disappeared, without a trace. Earth’s environment 3.8 billion years ago would have been completely different from today. First life would have arisen in the oceans and LUCA may have flourished next to geothermal vents on the seabed. Metabolism would have relied on and exploited chemical reactions in the sulphur rich environment. The development of photosynthesis harnessing sunlight and carbon dioxide to produce energy while releasing oxygen as a waste product would have to wait for a further 1.8 billion years. Cyanobacteria were the first organisms to utilise solar energy for photosynthesis. They flourished in the warm shallow waters of the continental shelves. At that time the atmosphere would have been composed of a highly reducing atmosphere of carbon dioxide, methane, ammonia and water vapour. Over the next 200 million years the highly successful cyanobacteria released oxygen thus changing the atmosphere to a more familiar composition with an oxygen level of around 20%. Oxygen is a highly reactive molecule and most forms of life could not adapt to this highly oxidising environment culminating in the Great Oxygenation Catastrophe. Most forms of life perished thus setting the stage for the radiation and evolution of life based on the successful cyanobacteria.

For me, the evolution of early life is a fascinating topic, but not as interesting as the original formulation of life itself. When we find evidence of early life in the rocks, that life is already highly complex and highly evolved. But going further back in time, what constituted the first ‘life’? And more importantly, if the first primordial life could somehow be resurrected in a ‘Mad Scientist’s test tube or surreal dream, would we recognise it as such? Now that IS a question.        

LUCA