Question: Why are we made up of cells

David Walker-Sünderhauf answered on 6 Mar 2019:

This question can reach way back to how life evolved in the first place, which I find really interesting (we had to do a presentation in school every year, one of mine was on the evolution of life through molecular evolution). So: like all other animals, we are made up of cells. This is because we all share a common ancestor, hundreds of millions of years ago, that evolved multicellularity. Before this, all life was single-celled. If however a single cell divided and then shared its responsibilities between other cells with the same genome, this might have made it “fitter” than other single cells that didn’t do this, helping this primitive multi-celled organism to propagate its DNA and gradually evolving more and more complexity and specialisation between different cells.
So we are made up of cells because, years ago, animals evolved from simpler single-celled organisms. But why were they “cells” in the first place?
Very briefly, the theory of molecular evolution explains how simple molecules might have self-assembled, and made crude copies of themselves. The more effective a molecule was at doing this, the more copies there would be of it – leading to a precursor of modern-day DNA or RNA. At some point, some of these molecules probably found their way inside a lipid droplet – and being inside a primitive membrane like this would give them a great selective advantage over other molecules, as they could make copies of themselves faster. This is probably how the first primitive cells formed, which eventually evolved into prokaryotic and then eukaryotic life, and then into multicellular organisms, animals, and humans.