Og

I want to tell you guys a little bit about how we know the evolutionary past of animals.

First off, we know that the earth is very old and that life formed a long time ago. This is from the fossil record and radioactive dating. We know the age of the universe through cosmology, interferometry and a variety of techniques that let us understand the age of things we look at when we peer into space.

Given that we understand the age of the systems involved and a general framework for when life arises based on fossil record, we can start to talk about the tree of life.

All life on earth uses DNA to encode proteins. All life is cellular in nature to some degree.

We use what is called "Homology" or genetic similarity. For example, if you look at something like a protein in the membrane of a cell that acts as a valve that can let calcium into a cell (called an ion channel) we can look at the genetic code that creates this protein.

For a little background, a protein is a series of amino acids (there are only 20 or so). Each set of 3 base pairs in your genes encodes a single amino acid. So, as your cells read the DNA, three base pairs at a time, it creates a chain of amino acids. ALL proteins are created this way. Your body is made of nucleic acids, proteins, fats, and sugars. And a few other trace elements. But a cell in your body is utterly those 4 types of molecules in complex patterns.

So what we can do is take the DNA of TODAY's life and look at how different their, in this example, ion channels are. Virtually all life uses ion channels to communicate between cells. We can then look at difference in the sequence.

The cool thing is that mutations happen at a fairly constant rate throughout time. Most mutations cause little to no change in a gene. But what these mutations can do is tell us how different two genes are in two organisms from one another. We can then talk about how closely related the components of organisms are and relate that directly to how they diverged over time.

For example, at some point in the distant past, channels that allow calcium to flow into your cells mutated and copied themselves into channels that allow sodium to flow into your cells. The channels that allow these two ions through are virtually identical except for a few small changes in their code.

By looking at the divergence across species and knowing a little bit about the environment of earth (from geology and cosmology), we can project how long two species have diverged from one another (i.e. how long ago did their common ancestor exist).

This divergence of species is principally how the age and relationship of organisms are measured. It's by comparing the sameness of their genomes.

Genetics really allowed for the map that illustrates the path of evolution.

This is how we know much about the history of life. It's not just the fossil record.

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