Where did cells come from?

A eukaryotic cell is a complex structure that includes nucleic acids, which hold information, as well as proteins, sugars and fats. But how did something as complex as this evolve? Barry Gibb gets to grips with the brief history of the cell, showing how the right molecules combined with protective membranes eventually led to the cells that make up all of us.

If the universe originated with the Big Bang, life on Earth started with more of a whisper. There are different ideas about how life got going: did that first glimmer of life originate from colliding meteorites, thermal vents in the young oceans, or the fabled "primordial soup" - a handy cocktail of organic molecules?

Making molecules

What is certain is that, at some point in evolution, larger organic molecules, such as nucleic acids and amino acids, began to form, aided by the volatile climate of the time. Scientists can only speculate on how many different combinations of molecules were rejected by natural selection before molecules that had the qualities needed to persist evolved.

What we now know about RNA, ribonucleic acid, makes it a strong contender for possibly being the first life on Earth. We tend to think of RNA as the industrious go-between that links DNA and proteins, but RNA is special. Its ability to act as a self-replicating molecule and perform catalytic reactions (like enzymes) makes it a biological Swiss army knife, showing the sort of self-sufficiency needed in an ambitious molecule.

Making membranes

Next in the evolution of the cell came the involvement of a protective barrier, something that could protect the delicate 'life' within and also provide a micro-environment sheltered from external forces. Certain fatty molecules, or lipids, have just the right properties needed to form this kind of protective membrane.

These membranes were more than just a barrier though, as the evolution of membranes afforded a more compact environment, enabling newly created organic molecules to be kept locally for other uses. Essentially, these were the first ancestors of prokaryotic cells: nucleic acids wrapped in a membrane, with little else.

Making mitochondria

According to microfossils (fossilised organic remains that need to be viewed with a microscope), the more complex eukaryotic cells arose somewhere around 1.45 billion years ago. The interior of these cells were vastly more organised than prokaryotic cells. Eukaryotic cells also possessed the ability to handle oxygen - vital in the increasingly aerobic atmosphere of the time. This ability came thanks to the inclusion of organelles called mitochondria (in animal cells) and chloroplasts (in plant cells).

There's no single explanation for how this situation arose. One contending theory is that eukaryote-like cells, without mitochondria, evolved and then somehow engulfed a form of mitochondrial predecessor, allowing these new hybrids to survive the changing climate.

Another idea is that early mitochondrion-like life forms evolved and invaded a prokaryotic ancestral cell that went on to evolve eukaryotic characteristics. These competing theories deal with what's known as 'endosymbiosis' - the equivalent of two businesses merging to their mutual benefit.

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Image: Eukaryotic Cell. Credit: Anna Tanczos, Wellcome Images.

This article is part of the exclusive online content for ‘Big Picture: The Cell’. Find out more about the ‘Big Picture’ series.