I just can’t resist writing about a terrific joke in a twitter exchange that touched on evolution and cryopreservation – one of the key features of our long-term evolution experiment (LTEE).

It starts with a New York Times article, by Carl Zimmer, about animals evolving in response to environmental changes caused by humans: “As Humans Change Landscape, Brains of Some Animals Change, Too.” The story features work by Emilie C. Snell-Rood, who analyzed changes in the brain size of small animals (mice, etc.) using skulls from museum collections. The story ended on this point: “What would be really cool would be to raise populations from 1900,” said Dr. Snell-Rood with a laugh, “but we can’t really do that.”

Now here come the tweets (with links removed, typos corrected, words fleshed out):

Carl: My new Matter column for @nytimes: Are some animals evolving bigger brains as they adapt to a human-dominated world?

Me, quoting from the article: “… would be really cool … to raise populations from 1900,” said Dr. Snell-Rood … “but we can’t really do that.”

Me again: And that’s why some of us love working with bacteria because we can have a viable frozen fossil record like in the LTEE

Carl: Too bad bats and mice aren’t like water fleas that you can revive after decades

Michele Banks: BREAKING Carl Zimmer supports plan to create zombie mice, bats.

Carl: Who doesn’t?

Michele: Of course Richard Lenski might be a zombie himself. IIRC, he was killed in the second act of Eugene Onegin, and yet HE LIVES.

Me: Oh no, my secret has been revealed. Luckily, I have backups in the freezer ;>)

Mark Martin: “Many are cold, but few are frozen,” after all.

Dear reader, I hope you find that funny. It’s especially funny to me because Mark’s joke matches exactly how we use the freezer in the LTEE project.

The LTEE has 12 populations of E. coli bacteria living in small flasks filled with, in essence, sugar water. Every day, we remove 1% of the culture volume from each flask and transfer that 1% to a new flask with fresh medium.

What happens to the other 99%? Well, they are discarded – we’d be drowning in bacteria if we let them all propagate. The populations grow by 100-fold each day, and the LTEE has been running for over 25 years.

However, we don’t discard those leftovers right away. They serve as a short-term backup in case there’s a mistake, like a broken flask before the next transfers are performed. So where do the 99% go?

Many are cold …

They get popped into the refrigerator, all of them, except …

Except every 75th day. On those days, after we move 1% of each populations into a new flask, we add glycerol – a cryoprotectant – to those that are left behind. These lucky cells are placed into a deep freezer for future study and permanent storage.

… but few are frozen.

Two final thoughts:

It’s interesting that The Left Behind are some of the lucky ones in the LTEE, no? The most fortunate ones, however, are those that just keep on evolving!

So, dear reader, count your own extraordinary blessings that you are the product of an amazingly long sequence – billions – of successful parents. Not a single one of your ancestors failed to survive and reproduce!