It’s been a long time in coming, but here’s Chapter 7. First let me say that these reviews are becoming more infrequently. There are several reasons, but not least of these is that the chapters are becoming much more involved and it’s difficult to snip out bits, because they are all important. Note, there is less of the actual book in this chapter review and I’ve added material to improve the discussion.
Adventures in Bodybuilding
No, we don’t mean Mr. Universe body-building. What we mean is, how did multi-cellular bodies originate?
Everyone should know that organisms come in two basic sizes. Single celled organisms (of the Domains Eubacteria and Archeabacteria) and multi-cellular organisms (of the domain Eukarya). Most every organism we see in our day-to-day lives are eukaryotes. Trees, mushrooms, ants, flies, dogs, people, etc are all eukaryotes. That is, they are all (with a few exceptions) multi-cellular and all have membrane bound organelles.*
It is well known that life on earth appeared between 3 billion and 3.8 billion years ago. [That single paragraph of Wikipedia links to 6 peer-reviewed articles.] This was not the first life, but the earliest life we can verify that was actually present. It’s pretty hard to see bacteria in rocks that are about one quarter of the age of the universe.
There is some debate about when multicellularity arose, but it was probably sometime between 1.5 and 1.8 billion years ago. Again, that’s the earliest evidence that we have found, not the actual start of multi-cellular life.
There is one very important reason that there isn’t much evidence of multi-cellular life before then. It has to do with energy. Multi-cellular bodies are big energy hogs. One portion of the body has to generate enough fuel to power all of the cells in the body. Think of a tree with that huge canopy to provide sugar to all the cells that can’t photosynthesize. And that tree doesn’t have to move or keep itself warm. Other bodies that move, hunt, and keep themselves warm even when the outside (the body) temperature is below freezing have to use a lot of energy.
Without oxygen, that level of energy just isn’t available. I won’t go into the details, but a cell that can use oxygen to make energy usable can produce almost 20 times the amount of useful energy as cell without oxygen (again, there are exceptions, but that’s the basics).
So for organisms with multiple cells, if there are too many cells that can’t get to the incoming food and there’s no way to transport the food to the interior cells, then the cells on the inside die.
OK, step 1, forming a body in the first place. Here’s where science actually has an answer. How? Because it’s been observed in the lab. Martin Borass performed an experiment in which he grew algal cells for a thousand generations… then introduced a predator. Within 10-20 generations he had masses of algal cells held together by proteins and other cellular produced compounds. Within another few tens of generations, the size of the proto-bodies was stable at 8 cells per body. This was big enough that the predators couldn’t eat the cell mass and small enough so that all 8 cells could get sufficient nutrients.
What’s really interesting is that the 8-cell configuration was retained indefinitely, even without the predator present. The cells had formed the first proto-body.
Of course, we’re a long way from specialized tissues… or not really. Here, we introduce Volvox.
Here we have a mass of 50,000 cells (give or take a few). Volvox (genus) is estimated to have evolved about 200 million years ago. This is basically a colony of green algal cells, EXCEPT that the cells have begun to specialize. Some are reproductive cells, some are cells for movement, it even has primitive light receptor cells.
So you see, like many other biological systems, bodies arose via redundancy. What I mean by that is that if you have two things doing the same thing, then one can change (evolve if you will) and the other can continue the original task. In Volvox, the cells have begun to specialize allowing for cells that are more efficient at movement to help the cell move, while other cells are more efficient at photosynthesis, providing food for other cells in the colony. This is more than a colony, it’s a body. It’s still considered a ‘colony’ in the scientific sense because of the lack of tissues and because all of the cells can form a new Volvox. But that’s a minor quibble.
Look at it this way, what happens when you cut a Volvox in half? You get two smaller Volvox. What happens when you cut a human in half? You get to piles of dead human. We are extremely specialized, to the point none of our cells can long survive without the others. Volvox and other semi-colonial organisms can.
Now, what’s really interesting, is that the some of the materials that we use to hold our bodies together are found in bacteria. Those proteins that hold us together have been around for almost the entire history of life on this planet.