A History Of Life, Part IV

The evolution of eukaryotic cells is thought to have occurred 1.5 billion years ago or so. This led to an enormous diversification and innovation. The pace of evolutionary change quickened. Eukaryotic cells diversified into a broad range of single-celled organisms. Most strikingly, some lineages of eukaryotic cells gave rise to true multicellular organisms. The evidence for when multicellular organisms first appeared is conflicting. If we look at the molecular phylogenetic studies, they suggest that the first multicellular organism must have lived as long as 1.5 billion years ago. The oldest fossil that we could argue as truly multicellular, however, is about 1.2 billion years old. More convincing fossils don’t appear until as recently as 600 million years ago.

When multicellularity arose isn’t clear, but its significance is much clearer. It enormously expanded the potential for evolutionary diversification that organisms could explore. It did so for a number of reasons. First, a single organism could now be composed of many cells, each of which could specialize for a single function. In other words, selection could act in different ways on different cells so that they could become more specialized. Cells could do just one thing very well within the amalgamation of cells in the organism.

The evolution of multicellularity also allowed organisms to become larger, much larger. Through the first couple of billion years of evolutionary history there are few if any organisms that you can see without a microscope. Once multicellularity evolved, organisms could become very large. In doing so not only could they become large complex things like blue whales, but more significantly and fundamentally, they could create an internal environment. This environment within the amalgamation of cells was much more favorable than the harsh external environment that they otherwise would live in. This allowed the specialization of cells on the outside of the organism to deal with harsh environments.

Perhaps the most significant consequence of multicellularity was that certain cells within an organism’s body had to become specialized for sexual reproduction. With single-celled organisms all we need is mitotic division. The evolution of specialized reproductive cells was something that had to happen, though, when we have a number of cells with different specialized functions. Some of these cells had to be sequestered and saved specifically for the task of making more individuals. Also, a different kind of division process must have evolved. This is the process of meiosis, which creates enormous potential for genetic diversity through recombination.

With the advent of sexual reproduction and the genetic variation that is a consequence of it, there was an enormous opportunity for evolutionary change.

If we reduce the entire history of the planet Earth to the scale of a single calendar month, the appearance of multicellular organisms would occur somewhere around the 24th day. Most of the month has already passed by the time has made it past single cells. From this point on, however, things accelerated very rapidly. Life increasingly became adapted and diversified into the untold forms that we see today.

There’s so much we can talk about in this regard, but I’ll leave that for another time.