Written by: Richard Lenski
Primary Source: Telliamed Revisited
The long-term evolution experiment (LTEE) began in 1988, and the E. coli populations are approaching 60,000 generations. That’s a long time for an experiment, and I hope it continues for much, much longer.
But when I give talks about the LTEE, I also try to remind people that 26 years is only a drop in the proverbial bucket of evolutionary time. If you were to add these experimental populations to the tree of life—or even to a tree showing only other E. coli strains—they would not be visible to the eye because the branches they represent—tiny twigs, really—would be so short (in time) and so close (in genetic distance) to their ancestors.
On Time and the LTEE
Life has existed on Earth for roughly 3.5 to 4 billion years. That’s about 140 million times longer than the LTEE has existed. Expressed the other way around, this experiment has been running for about 0.0000007% of the time that life has been evolving on our planet.
As a said, a mere drop in the bucket of time …
That’s a somewhat mixed metaphor, though, with “a drop in the bucket” being a statement about space and relative volumes, not about time. And that got me wondering about the spatial scale of the LTEE relative to the spatial scale of the biosphere.
If the LTEE is just 0.0000007% as old as life on Earth, what fraction of the space—of the total biovolume—of life on our planet exists in the confines of the LTEE?
On Space and the LTEE
That’s a harder a question to answer. We know the volume of the LTEE: there are 12 flasks, one for each of the evolving populations, and each flask contains 10 milliliters (mL) of liquid medium. (In medicine, by the way, a drop has been defined as 1/20th of a mL, so each flask in the LTEE contains 200 drops.) If we sum across the populations, then the LTEE occupies 120 mL.
Before you read further: What’s your quick intuition? Is the LTEE larger on this spatial scale than on the temporal scale? Or is the LTEE smaller?
Volumes and Numbers
How should we estimate the volume of Earth’s biosphere? Here are three back-of-the-envelope approaches to get a rough sense of the scale.
1) Most of the Earth is covered by its oceans, which are full of life. While life is not equally abundant throughout the oceans, none of that space is entirely devoid of life. The total volume of Earth’s oceans is about 1.3 billion cubic km. That’s a lot of mL! A mL is a cubic centimeter, or cc, and that’s 1/(100^3) = 1 millionth of a cubic meter. A cubic meter is 1/(1000^3) = 1 billionth of a cubic kilometer, and the oceans contain over a billion of those cubic kilometers.
So the 120 mL in the LTEE correspond to 120 / (1.3 x 10^9 x 10^9 x 10^6), or about 9 x 10^-22 of what the oceans contain. That’s just 0.000000000000000000009% of the volume of the oceans.
By this calculation, then, the temporal scale of the LTEE is ~75 trillion times greater than its spatial scale, when both are expressed relative to nature. If the LTEE is “a drop in the bucket” with respect to time, then that drop has to be diluted by a factor of 75 trillion with respect to the oceans.
2) Let’s try another quick-and-dirty calculation. Most life, in the oceans and on land, is near the Earth’s surface. The surface area of our planet is about 510 million square kilometers. If we take just the top meter, that’s equivalent to 510/1000 = 0.51 million cubic kilometers. That’s about 1/2600 of the volume of the ocean. But even this conservative estimate of the volume of the biosphere makes the relative scaling of the LTEE with respect to time and space differ by a factor of 30 billion.
3) Here’s one more approach—it’s based not on the volume of the physical environment but, instead, on the number of organisms in the LTEE and in the biosphere. When grown to stationary-phase density in the LTEE environment (i.e., when the limiting resource, glucose, is depleted), the ancestral bacteria could achieve a maximum density of ~5 x 10^7 cells per mL. Most populations have evolved so that they now produce slightly fewer, but larger, cells; and one population has evolved the ability to use the citrate that is also in the medium, and it now reaches a density that is several times greater than the other populations. In any case, given 10 mL of medium for each population, and 12 populations, the total population size across the LTEE is on the order of 10^10 cells.
And how many cells exist in the Earth’s biosphere? Whitman et al. (1998, PNAS) estimated that there are more than 10^30 prokaryotes—bacteria and archaea combined—in the biosphere, and they make up the great majority of all living things.
So by this approach, using the number of cells as a proxy for the spatial scale, the size of the biosphere is over 10^20 (a hundred-million-trillion) times larger than the LTEE. We’re back into the trillions in terms of the relative scaling of the temporal and spatial scales of the LTEE.
On Time, Space, and the LTEE
By all three approaches, then, the LTEE is vastly older with respect to the history of life on Earth than it is large with respect to the size of Earth’s biosphere.
The LTEE really is a long-running experiment, as experiments go.
But the LTEE is a “drop in the bucket” with respect to how long life has been evolving on Earth. And it is a vastly more miniscule “drop in the bucket” when compared to the spatial extent and number of living organisms on our planet.
Maybe I should give the LTEE a new name—the “incredibly tiny but relatively long-term evolution experiment.”
[Photo of a water drop on a leaf taken by tanakawho and shared on Wikipedia (en.wikipedia.org/wiki/File:Water_drop_on_a_leaf.jpg).]