Living things are made of four primary components – DNA, proteins, carbohydrates, and lipids (fat).

Just like molecules are collections of different types of atoms, living things are made up of cells.

A cell is a tiny bag filled with DNA, proteins, water, and other substances. The bag enclosing the cell is made up of fat, also called lipid.

You can think of DNA as a recipe book. Each recipe tells how to make a certain kind of protein.

Proteins are the workhorses of the cell. They are tiny machines that can do all sorts of wondrous things, including making copies of the DNA when cells get ready to divide.

But if DNA specifies the recipe for proteins and if proteins are needed to make copies of DNA, how did the whole process get started?

The prevailing view is that life probably started at warm vents in the ocean floor.

These vents provide two things that life desperately needs – warmth and a source of electrons. The latter comes from electron-rich hydrogen gas that bubbles up from inside the earth.

The difference between the electron-rich water in the vents and the surrounding electron-poor ocean water is called a gradient.

We can think of the vent water as having more potential energy, just like a ball sitting at the top of some stairs. By moving downhill, the ball converts potential energy into the energy of movement (kinetic energy).

Early cells might have exploited this difference as a primitive sort of metabolism. This might have allowed them to evolve their DNA recipes before they actually needed many proteins.

The minerals in the warm ocean vents might also have provided tiny “caves” to enclose these would-be cells until they figured out how to make their own fatty membranes.

Once cells enclosed their contents in fatty membrane bubbles, they were free to leave the warm vents and populate the vast ocean.

However, they needed to come up with a way to replace the passive energy they got for free from the electron gradient in the vents.

So they invented a simple pump to push protons (H+ ions) out to create their own gradient. And they invented a little turnstile that could let the protons back in but use the energy to make ATP.

We can tell that all cells in all living things came from a single type of ancestral cell because all cells are remarkably similar in their overall construction.

This single type of ancestral cell is often referred to as LUCA which means “last universal common ancestor” – the mother of all life on earth today.

Just think of it. Every cell in your body as well as those in every plant, animal, and microbe descended from LUCA.

Scientists estimate that LUCA arose roughly 4 billion years ago, or about a half-billion years after the formation of the earth.

By looking at cells from different forms of life to see what they have in common, we can get a pretty good idea of what LUCA was like.