*life
In: Earth Science
Our best current hypothesis goes like this:
* The building blocks for RNA can be found in nature. Given enough time, those building blocks assembled into strands of RNA by pure chemical means by chance.
* RNA is unusual in that it can both be replicated (that is, it works as a genetic material) and can cause certain chemical reactions to happen faster (that is, it can play the same role as proteins do in modern biology). This led to some RNA strands catalyzing reactions that made those RNA strands reproduce or be more stable in their environment. These RNA strands became self-reproducing and exercised control over their environment in a way that depended on their structure, meaning that they were arguably the first living things. Examples of these isolated RNA “organisms” exist today in the form of plant diseases called [viroids](https://en.wikipedia.org/wiki/Viroid).
* Structures similar to cell membranes can form without the intervention of life, by purely chemical means. Some of those membranes eventually wrapped around a strand of RNA from the previous step. This allowed the RNA to work within a controlled environment.
* These membranes-around-RNA-strands (what you could loosely call the first cells) figured out how to duplicate their RNA and to split the membrane, in such a way that they maintained control over their internal environment. At this stage, cells (and not RNA strands by themselves) became the fundamental building blocks of life.
* A specific kind of RNA, what we’d now call tRNA, developed that could use RNA as a template to build proteins. This was better than the RNA doing things itself, because a single authoritative copy of the RNA could be kept and used as a template for thousands of proteins. At this point, proteins became the main (though not the only) functional molecules within a cell, with RNA taking a back seat in the role of pure genetic material.
* DNA’s building blocks can also be found in nature. DNA is a bit harder to assemble, and more importantly can’t directly “do things” like RNA can. Because RNA can cause the formation of other RNA or proteins, it could create a new type of RNA that “copies” information off of DNA (what we would now call mRNA). Because DNA is much harder to damage than RNA and can be more easily repaired because errors can be detected from mismatch between its strands, DNA became the main way to store information in cells. RNA was now neither the main genetic material nor the main functional molecule, and instead took on a role as an intermediate between DNA (genetic material) and proteins (functional molecules).
It’s around this point that the last universal common ancestor – the single cell that would ultimately produce all cellular life on Earth today – lived. All cells known today:
* use DNA as their genetic material,
* use DNA to produce RNA according to a specific mechanism,
* use that RNA to produce proteins through a code that is almost universal among living things (a very few organisms later evolved slight modifications),
* and use those proteins to carry on the functions of life,
* within a cell membrane of phospholipids that separates the cell from its environment.
Other kinds of life probably existed at this time, but none of their descendants have survived to the present day. Note that modern cells actually do carry remnants of this very early world, with [RNA acting as a catalyst](https://en.wikipedia.org/wiki/Ribozyme) in specific parts of even modern cells.
The theory I’ve summarized above is the “RNA world” hypothesis. It’s generally the best-accepted theory today, but there are a number of competing theories that are also possible; it’s possible we’ll never truly know the answer to this question. Our best guess is that this process happened over a few hundred million years, beginning somewhere between 3.5 and 4 billion years ago (shortly after the formation of the Earth), probably at volcanic vents in the ocean.
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