Your Cat Is Your Cousin
Why the Family Tree Is Far Bigger Than It Seems
That floof next to you is your cousin.
Your literal, actual, flesh and blood cousin.
This is not a joke or an exaggeration. More specifically your cat is approximately your 4.6 millionth cousin, 18.4 million times removed.
This is because of the way evolution works. You and your cat have a common ancestor about 90 million years ago.
Roughly speaking, each generation adds one degree of cousinhood. Humans and cats are believed to have had a common ancestor that lived about 92 million years ago. If each generation lasts about 20 years, then the degree of cousinhood is 92 million divided by 20 which equals 4.6 million.
What actually is a cousin?
Cousin means belonging to the same family, but not in the same direct parent-child line. A cousin is referred to as removed if they are separated by generations, for example cousins from your parent’s generation are once removed.
See the diagram below for a simple visual explanation of how cousins and degree removed is defined.
The overall evolutionary picture
In fact a similar statement can be made for all cellular life forms on earth. Scientists believe that all cellular life on Earth is descended from a single common ancestor that lived 4 billion years ago. That means that all subsequent life (including bacteria, plants, and animals) are, in a very literal sense, related.
The closeness of the relationship in terms of degree can be expressed similarly as how it was done for the cat, using the approximate number of generations in the past (the nth cousin), and the difference between the number of generations of each branch (times removed).
So using a table that mirrors the first cousin table shown earlier, the human-cat relationship is merely an extrapolation of the family tree we are all so familiar with.
Putting some numbers on this, with estimates for several representative species, gives an idea of the overall situation.
| Lifeform | Time since most recent common ancestor (million years) | Length of a generation (years) | Cousin degree from human (million) | Times removed from human (million) |
|---|---|---|---|---|
| E. coli (bacteria) | 4200 | 0.0027 | 210 | 1532790 |
| Algae (simple plant-like organism) | 1500 | 1 | 75 | 1425 |
| Banana | 1200 | 1 | 60 | 1140 |
| Cactus | 1200 | 1 | 60 | 1140 |
| Python | 325 | 15 | 16.3 | 5.4 |
| Cat | 92 | 4 | 4.6 | 18.4 |
| Dog | 87 | 4 | 4.4 | 17.4 |
| Punch (Japanese macaque) | 23 | 10 | 1.15 | 1.15 |
| Harambe (gorilla) | 9 | 20 | 0.45 | 0 |
| Chimpanzee | 6.5 | 20 | 0.325 | 0 |
The farther back the shared ancestor, the more distant the relationship
*How degree of cousinhood and times removed are calculated: Using the human-cat example, the most recent common ancestor is estimated at about 92 million years ago. Using a 20 year human generation, this corresponds to about 4.6 million generations (the degree of cousinhood). Using a shorter 4 year generation for cats gives about 23 million generations; the difference between these, about 18.4 million, is the number of times removed.
So admittedly, these lifeforms are very distant cousins but cousins nonetheless.
Bacteria are an extreme case
E. coli is the most extreme example shown because it separated so much earlier from the human evolutionary branch than the other examples. Also, having such a short generational life span dramatically increases the degrees removed. But even there we are cousins.
How do we know this is true?
How sure are scientists that all these lifeforms came from the same common ancestor? Based on current evidence, the scientific consensus is yes, amazing as that sounds, it’s true. This conclusion is based on the coding contained in the DNA. DNA is passed down from generation to generation, changing from time to time due to mutation but keeping much of the same fundamental chemical configuration.
DNA contains the code, basically an instruction manual for building proteins that all together form a living organism. Within the code are instructions for various functions such as starting or stopping the production of a specific protein. All life uses much of the same exact coding down to the molecular level. The specific coding is in large part arbitrary, there are many subtle differences in the DNA code which would work, but only one version is found in all DNA. It would be extremely unlikely for this exact system to have developed independently multiple times so the conclusion is all DNA today originated from a single source, an ancestor.
The LUCA
This earliest ancestor is called the last universal common ancestor or LUCA. LUCA, which would have been the simplest of single celled organisms, is estimated to have lived about 4 billion years ago.
Human populations are closer cousins
Contrary to the very distant relationship between species, most human populations are much more closely related. Estimates suggest that among all humans worldwide we are probably no more than 150th cousins.
The reason is that the number of ancestors grows rapidly with each generation. We all have 4 grandparents, 8 great grandparents. But just 10 generations back there are over 1024 ancestors, and by 20 generations that number reaches about 1 million.
Within the same region or country we are even more closely related
Within the same large region, we are even more closely related. For example, within the indigenous population of Europe, people may be no more than 30th or 40th cousins corresponding to a common ancestor roughly 1,000 years ago.
In smaller or more isolated groups the relationships are even closer, often in the range of 5th to 10th cousins as seen in populations such as Iceland, Polynesia, and the Amish in the United States.
Not everything that moves is a cousin
There are some non cellular biological forms that are not in our lineage, for example viruses. So you are not a blood relative of the COVID or influenza virus. That should provide some comfort.
Epilogue
My cousin the cat is enthusiastically chowing down on its cousin, tuna fish packed into a can. Now please excuse me while I do the same with my cousin, an ear of corn. At least the salt isn’t a member of the family.