Living pictures of evolution past – What an Enigma!
This is the third instalment of three posts, Living pictures of evolution past, that use living biological weirdness to illustrate key processes in the early evolution of life.
So you’re eating lunch. Spaghetti. But then, you have a brilliant idea! Rather than digest this food, why not use it? It would make a great wig. So you put it on your head.
This is what one organism, called ‘Enigma’ does, but for a body part much more vital than hair. Turning potential food to other uses is a neat biological trick, and was an absolutely vital step of evolution.
We have previously talked about amazing examples of the development of multicellularity and the forming of organs. But long before this step, single celled organisms were working on their own organs, called organelles.
Your cells contain little organelles called mitochondria. They supply usable energy to the cell. Other animals, fungi and plants have them too. The spectacularly weird thing about them, and a huge clue as to how they appeared in the ancestor we share with fungi and plants, is their DNA. Mitochondria have their own DNA, separate from our genome.
This DNA, and the mitochondrion as a whole, looks suspiciously familiar. There’s a similar story for the organelle plants have to photosynthesise, the chloroplast. Both these organelles look exactly like bacteria.
This observation led people to come up with the idea of endosymbiosis. It goes like this: billions of years ago, a cell is having lunch. But, for some reason, instead of digesting the bacterium (proteobacterium to be precise), it keeps it alive. This bacterium produces useful energy for the cell, giving it an evolutionary advantage. Over generations, most of the genes for the bacterium are moved to the cell’s nucleus, but a few remain inside what is now the mitochondrion.
The same thing happened again, when a cell with a mitochondrion ate a cyanobacterium (a bacterium that can photosynthesise) and thus the plants were born.
But how do we really know this happened? How do we even know it is possible? This is where Enigma comes in. Called Hatena arenicola (Hatena means something like enigma in Japanese) this single-celled creature performs the trick every generation!
Hatena ingests and keeps a green alga, which it uses for a chloroplast, allowing it to live like a plant. When our cells divide, each new cell gets some of the mitochondria that were in the parent cell. But when Hatena divides, there is only one green alga available, so one of the two daughter cells misses out. But the one that misses out it doesn’t have to live forever like a non-plant, who has to eat things to survive. It can can eat a green algae of its own. When it does, it gains the ability to be plant-like, it swims toward light, and loses its ability to eat. It is repeating the process of endosymbiosis that happened so long ago, right before our very eyes!
Major milestones in the evolution of life happened so long ago they are tricky to investigate. So it is fantastic that we can see some of these key early steps of evolution, long past, mirrored in characters like Dicty the slime mould, the Frankensteinish Bluebottle, and that enigma Hatena.
These, of course, are the (delightfully weird!) exceptions; the overwhelming majority of biology that surrounds us now was nudged into shape by myriads of almost imperceptible changes. Slowly, slowly, these accumulate, delicately shifting the forms of life into what we see today. Now is a snapshot of an immense continuum of life, that was not like this in the past, and will not be like this in the future.