From Building Blocks to Self-Replication
So, given that there is no shortage of organic molecules, how then do we get things to self-replicate? And what were the first self-replicating molecules? The scientific consensus is now the RNA molecules were likely the first replicating molecules. We have to keep our discussion of this topic fairly brief, but if you would like to explore it in more detail, visit the "Exploring Life's Origin Project".
Back to RNA, what is it? To understand more about RNA we turn to this site,
RNA, which stands for ribonucleic acid, is a polymeric molecule made up of one or more nucleotides. A strand of RNA can be thought of as a chain with a nucleotide at each chain link. Each nucleotide is made up of a base (adenine, cytosine, guanine, and uracil, typically abbreviated as A, C, G and U), a ribose sugar, and a phosphate.
The structure of RNA nucleotides is very similar to that of DNA nucleotides, with the main difference being that the ribose sugar backbone in RNA has a hydroxyl (-OH) group that DNA does not. This gives DNA its name: DNA stands for deoxyribonucleic acid. Another minor difference is that DNA uses the base thymine (T) in place of uracil (U). Despite great structural similarities, DNA and RNA play very different roles from one another in modern cells.
This article has a nice discussion outlining how scientists went from suspecting RNA to deciding that it was indeed the most likely candidate as the first replicator.
But before we can have RNA replicating, we need to make the specific building blocks of RNA and there has also been considerable discussions on that topic. A famous experiment in 1952, the Miller Urey experiment recreated an early Earth atmosphere and synthesized complex organic compounds such as amino acids via lighting in a bottle (electrical sparks). Their starting composition consisted of very simple organic compounds, namely water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2). The implication is that a primitive earth has all the ingredients to make life.
In 2015, NASA scientists simulated space-like conditions and found that they could produce many molecules that were RNA/DNA components. The implication is that interstellar space may be a much more fertile environment than anticipated. This finding has significant implications for the possibility of life beyond Earth. This is what some of these molecules looked like:
As one of the scientists put it
“"Nobody really understands how life got started on Earth. Our experiments suggest that once the Earth formed, many of the building blocks of life were likely present from the beginning. Since we are simulating universal astrophysical conditions, the same is likely wherever planets are formed,"
Another implication of this finding is that you may not need the Earth to start life. Panspermia, the hypothesis that life exists throughout the Universe and is distributed by a variety of means has been debated for a long time. Astronomers such as Fred Hoyle and Chandra Wickramasinghe have contributed to it. It is beyond the scope of this module to comment on it, but check the Wikipedia page for some interesting implications of that hypothesis.
While we don’t know where life formed, we do know that it managed to establish a foothold on Earth very quickly. We have physical evidence of biogenic graphite dating from 3.7 billion years ago. Graphite is composed of carbon. Biogenic means that the carbon was concentrated using biological activity, which is determined by the isotopic composition of the carbon. The implication of this is that some form of life was concentrating carbon 3.7 billion years ago.
And there is now potential evidence of biogenic carbon, isolated from mineral grains (zircon) dating back to 4.1 billion years ago. We need to point out that we have in fact no rocks preserved from that age, thanks to Earth’s active plate tectonics. Only individual minerals survive from 4.1 billion years ago and they indicate that life may have existed. If you scroll to page 2 on the article, you will see that the evidence consists indeed of tiny specs in a tiny grain, but the implications are definitely not tiny. And note that the authors do use the word "potential" in the title.
Update Jan 2018 - There is an active scientific debate about when we can agree on evidence of life on Earth and what that evidence is. This recent article in Quanta magazine does a very nice job of summarizing some of the scientific arguments for early life on Earth and indeed on the debate of what the conditions of the planet might have been like. It goes into more details than we provide here - and it has pictures!, You can even download a pdf version of the article. If you find questions about early Earth interesting, I highly recommend reading it. Now, back to our regular programming.......
So, life was able to set up shop on a very early Earth, a planet with no free oxygen, a place on which we would not be able to live without a space suit. This bodes well for life in other parts of the solar system. To get a better idea of what conditions life can exist in we’ll now take a look at life on Earth in places you would not want to visit. We’ll examine the realm of the extremophiles, which refers to life that can exist in conditions that would be toxic or deadly to us.
But before we leave to examine extreme settings today, we need to mention that life is now responsible for making our planet into the habitable home it is today. The free oxygen we take in with every breath is produced by photosynthesis. Oceanic cyanobacteria were the first organisms capable of it and they, forever altered the atmosphere of this planet. This happened approximately 2.3 billion years ago and we refer to this momentous change as the Great Oxygenation Event (GOE).
One note to any biology majors who may be reading this: It is beyond the scope of this course to discuss the details or phylogenetic relationships of our single cell friends. So if you would like to investigate the possible roles of Archaea, Eukaryotes and Bacteria, you will have to do that on your own.
