> What I've always wondered is about cells. They split and multiply and grow, and if it all starts from this one little microscopic cell, where do all the atoms and molecules for what eventually becomes a full-grown human come from? They're obviously not packed into that one little cell.
>
> -Eric Sleator

A related question, and a much more difficult one to answer, is how the *first* cell came from. We have only observed cells coming about from splitting/combining existing cells. But somewhere along the line cells had to come into existence via a different means of production.

As with the chicken/egg question, the answer, I think, is that there was no one clear moment where a first cell came into existence.

The first step (or one of them) is the production of the amino acids and other building blocks that make up cells. This part we know the most about -- experiments have shown that these chemicals will be created from the raw elements that were present in the early days of the Earth, given the right conditions and enough time.

Among these chemicals would be phospholipids such as those that make up cell walls, providing for semi-permeability which is essential for cells to maintain their precise working conditions...but how did it come about that enough of these phospholipids grouped together to form a cell enclosure with the right chemicals trapped inside? We don't know for sure. We can make hypotheses about the possible lines of micro-evolution that were taken, but the magnitude of the experiment which would be required to actually go about recreating this process might take a laboratory as big as the Earth itself and a comparable amount of time (billions of years!) Sure, maybe we could watch carefully and shelter those cell-like structures that we see are on the right track, greatly increasing their chances of surviving, but then we're not letting things develop on their own.

Beyond the formation of the cell wall itself, of course, you need the complex chain of reactions headed by DNA which replicates the cell, and I suspect that this was *much* tougher to come about naturally -- perhaps quadrillions of potential cells failed before enough of them got it right and survived long enough to reproduce. Of course, the two processes of cell wall formation and replication are very much independent, so there's the added complication of evolving *both* of these processes and getting them to work simultaneously.

And of course you need other systems to develop simultaneously, such as mitochondria and other organelles necessary for powering the cell's work, but it seems to me that these would come about a bit easier.

So there you have it, perhaps. An immensely difficult uphill battle from chaos to get the chain of cell-to-cell creation started. There was probably no one moment when a first "cell", as we'd define it, was created. It was a mind-bogglingly slow process with a continuum of cell-like structures and encapsulated chemicals, some of which lasted longer than others, and some of which reproduced more often or more faithfully to the initial version than others.