There are a lot of statistics on the internet about rodent - human genetic similarity but most of those numbers are either wrong or misleading. But there is an important lesson about evolution here.

Fake News

For some reason, mouse data has gotten more publicity than rat data. This beautiful chart reports that mice are less similar to humans than chimpanzees are:

The New Scientist magazine gives a much higher value:

That value seems unreasonable given that chimpanzees supposedly have 98% the same genes as humans. Remarkably, CNN published a number that makes mice seem more similar to humans than chimpanzees are:

What’s going on?

Those statistics come from a misunderstanding of the genetic concept of ortholog and paralog. When a geneticist says a human and a rat have the same gene, that doesn’t mean that the genes are identical.

A given gene could be very different in rat and human but will be described as the same because it serves the same function. Globin genes in various species are very different from each other but they all serve to carry oxygen in the blood. They use technical terms to describe how similar these genes are: homolog, ortholog, and paralog. This chart shows the evolutionary history of globulin genes.

Percent similarity doesn’t have a set meaning

There’s other ways to define genetic similarity. You could look at percentage of individual nucleotides the DNA that are the same, genes having at least one change, genetic changes that actually affect function, order of genes on chromosome, and lots more. I discuss some of those elsewhere and point out that you can get anywhere between 29% to 99% similarity between chimpanzee and human, depending on how you compute the numbers.

Here’s some meaningful statistics

The following statistics come from four articles:

Many of the genetic differences between rodents and humans are unimportant.

The rate of genetic change has been higher in rodents than in primates, including a three-fold higher rate of change in “neutral DNA.” That’s changes in DNA that don’t have any effect on the organism. Interestingly, the rate of this type of genetic change has been a bit higher in rats than mice.

Rat, mouse, and rat genomes contain multiple copies of “immobilized transposable elements.” That’s a kind of parasitic genetic material found in most animals. You could call them truly selfish genes. They constitutes about 40% of the mouse and rat genomes, and almost 50% of the human.

A sizeable number of the genes that differ between rodents and humans are involved in smell (olfaction). Rodents have 1,200–1,400 genes for smell but people have a lot fewer.

Rats, mice, and humans also have a form of DNA called “large segmental repeats.” This is probably nonfunctional but makes up about 3% of the rat genome, 1–2% of mouse, and 5–6% of the human genome.

Important genes are similar in rodents and humans.

About 90% of rat genes are “strict orthologues” in both mouse and human genomes. Again, orthologs are genes that may have changed but still do mostly the same thing in rats and humans.

There are more than a thousand human disorders showing Mendelian inheritance. A very high fraction of them, possibly nearly all, have orthologs in rats.

There are very few genes that are present in one species but not another. One group of researchers looked at a sample of 731 mouse genes. They found that only 14 of them, less than 2%, didn’t seem to exist in people.

Mice and rats look similar but are genetically different.

The common ancestor of mice and rats lived about 16 million years ago. The common ancestor of humans, rats, and mice lived about 87 million years ago. (Picture taken from

The rate of genetic evolution has been higher among rodents than primates but much of this is neutral evolution involving changes that don’t affect function. Rodents have also had a higher rate of chromosomal changes than primates.

Lessons about evolution

Anybody who has studied anatomy or physiology should have noticed that mammals are made up of mostly the same parts.

To get a sense of that look at a rat skeleton.

And then look at a human skeleton.

You will see that the two species have mostly the same bones although they are shaped differently. That’s also true for internal organs; both species have the same organs. Again, rats and people have mostly the same biochemicals inside them.

So it isn’t surprising that rats and humans have mostly the same genes, although the DNA sequences differ.

Creationists have often pointed out that it is difficult for mutations to produce new functions.

Footnotes