A crucial step in the evolution of complex organisms is the appearance of multicellularity. Key questions are “how did multicellularity evolve? Did it evolve once or multiple times? How did cells make the transition from life as a solo cell to associating and cooperating with other cells such that they work as a single, cohesive unit?”
It turns out that the shift from a single-cell system to a multi-cell one is a fairly complicated process in which cells have to undergo changes in shape, function, structure, and development.
However, there are certain sets of requirements that must be met in order for multicellularity to evolve. These include that cells must adhere to, communicate with, and cooperate with each other, and that cells must specialize in their functions (i.e., that not all cells do exactly the same thing, otherwise they would just be a group of cells or a colony). In order to make these things happen, cells must not reject each other. In other words, they must be genetically compatible to some extent—analogous to how our human bodies reject foreign items that are not recognized by our cells.
But despite what seems to be quite serious obstacles to be overcome, it has occurred independently a surprisingly large number of times.
Defined in the loosest sense, as an aggregation of cells, multicellularity has evolved in at least 25 lineages. However, even when defined more strictly—requiring that cells be connected, communicate, and cooperate in some fashion or another—it has still notably evolved once in animals, three times in fungi, six times in algae, and multiple times in bacteria.
This shows once again that, like with the eye, what seems to be an insurmountable evolutionary problem can be overcome. (Richard Dawkins’s book Climbing Mount Improbable gives more such examples.)
You can see the paper based by Karl J. Niklas on which the above article is here.
I would argue that cell specialization is a different thing than basic multicellularity, with different advantages. For example, sea algae might stick together in strands in order to adhere to a rock and not be washed away by the tide (think stromatolite) and this does not require any specialization. Breaking things up like that makes it eaiser to come up with a Darwinian sequence -- many small steps, each being advantageous.
Myxobacteria are among my favorite examples of multicellularity. They demonstrate that a complex eukaryotic genome isn’t necessary to have a fully developed lifestyle and specialized reproductive cells. These bacteria are predatory swarming bacteria, and will devour anything they can find along their path. Once resources are depleted, they form fruiting bodies, which look like tiny twisted trees, with the “leaves” full of rapidly-dividing bacteria ready to get picked up and carried to new feeding grounds. [Image]
But this life cycle is genetically expensive. They have among the largest genomes of all bacteria, ranging from 9 to 13 million base pairs. Maintaining all that DNA is expensive. A cell with a large genome needs to be generating a lot of energy, and the bigger the ratio between energy generated and genome size, the better. Bacteria generate energy primarily at their cell membrane, so you’d expect bacteria with large genomes to be really large and have expansive membranes, right? Well, you do sometimes see that, but as a bacterium gets bigger and bigger, it runs into other problems. A single centralized genome must be responsible for replacing and maintaining all that energy generation machinery. So the more physically distant that machinery is from the genome, the less efficiently it will run.
A few bacteria have solved that problem by having multiple copies of their genome at various locations close to their membranes. But one particularly crafty species came up with a clever solution over 2 billion years ago: they engulfed several small bacteria with high membrane-to-genome ratios, and had a separate centralized, enveloped genome to control all other cellular functions. These are the first eukaryotes, and those engulfed bacteria became mitochondria. You can read more details here.
So energy generation efficiency became decoupled from genome size, and genomes could grow larger and larger. You would expect that in eukaryotes there is much less of a penalty for an extra large genome with lots of junk when compared with prokaryotes. And that’s exactly what you find. Eukaryotes have the same number or slightly more genes than most bacteria, but have far bigger genomes with lots of extra junk. But the ability to make bigger genomes also provided the ability for complex gene regulation systems, and that’s why eukaryotes comprise the large majority of multicellular life.
For further reading, I’m fond of this paper from 2007. It’s similar to the one linked in the OP, but much (MUCH) longer and more in-depth.
@invivoMark,
Man, these bacteria are smart. We should probably have more to fear from them taking over the world than chimps.
There’s a curiosity about multicellularity that nobody seems to have noticed very much. Most of the estimated 25+ times of evolution of multicellularity have been plantlike or funguslike or slime-mold-like. Animal-like multicellularity seems to have evolved only once, when the animal kingdom emerged from the choanoflagellates. It could have evolved from other animal-like protists like alveolates or ciliates or excavates, but it didn’t.
So is it difficult to evolve? Could other planets have lots of trees and mushrooms and slime molds but no animals?