Multicellularity: Life on Earth starts teaming up
People say, there’s no “I” in team;
but there definitely is an “I” in multicellularity.
Once life on Earth got going, it really got going. Bacteria ruled the entire planet very quickly, with nothing to compete against. However, there is only so much you can do with just one cell, even a fancy one with bells and whistles (see our Eukaryotes post). The next step is combining a few cells together to make something a bit more complex. This is called a “multicellular” organism – an organism with more than one cell. All the creatures we know and love, lions, elephants, trees are multicellular – its a massively important step in life on Earth evolving. Let’s dive in, to cells.
Single-celled organisms: A defense of keeping it simple
Once single-celled life emerged, the next obvious step is for a few single cells to combine. There are obvious advantages to being multicellular. If you just have one cell, that once cell needs to be able to do everything. However, if you have a few cells, these cells can specialize and change their structure to do different jobs. When a group of specialized cells combine together, they form what are called “tissues”, such as muscle and bone tissue. This means you can make start to make the structure that allows for complex and dynamic organisms – skeletons, muscles, skin etc.
However, we shouldn’t discount single-celled organisms. A lot of single-celled organisms are really wonderful, many of them thousands of times larger and more complex than bacteria. For instance, there are amoebas – fast-moving, predatory blobs that wrap themselves around and eat bacteria – they can also be pretty and really brightly colored. There are also paramecium, large single-celled organisms which often farm smaller single-celled organisms inside them. Your average drop of pond water has thousands and thousands of these creatures and there are millions of them covering your hands right now. There are even a few really weird single-celled organisms that are massive, 20 or 30 centimeters across.
How did multicelled organisms evolve? What were the first ones?
Multicellularity has evolved at least 25 different times on Earth. It evolved in slightly different ways each time, in different creatures for different reasons. Some creatures even evolved multicellularity and then, lost it, by becoming single-celled again. The whole picture of developing multicellularity is really complex and inconsistent. It is really hard to talk about.
There are a few things we can say about the first multicellular organisms. Firstly, they did not have complex structures like modern animals. They were more like big coordinated colonies of single-celled creatures., like slime molds and fungi.
However, just because early multicellular creatures didn’t have complex structures like bones and skin, doesn’t mean they weren’t capable of incredible things. For example, slime molds can solve mazes – they leave a sticky trail wherever they go, so they can tell which areas they have already explored. They farm bacteria, carrying small amounts of the bacteria with it and then depositing it in new areas, letting it grow for a while so they can come back and harvest it later. Perhaps most impressively, they “learn”, when exposed to a negative stimulus like salt, they will work out what concentration of the chemical they can survive, and remember where they were exposed and avoid it. We have no idea how they do this, slime molds are just mind-boggling.
Other early multi-cellular organisms did form some basic complex structures. For instance, Gonium, one of the most simple multicellular organisms, is basically a colony of algae that all imbed itself in one larger cell structure (see the image below). There are a lot of other structures like this – where several single cells bind themselves together in some way, and slowly begin to reproduce together.
There are lots of other ways to think about multicellularity evolving. Sometimes it was probably two different species, slowly evolving to like and work together, until they became so reliant on each other they weren’t able to operate independently. Other times, it was probably a single-celled creature that evolved separate partitions, so it started to split itself into separate divisions and became multicellular. We’ve actually even managed to make some single-celled organisms evolved some of the characteristics of multi-cellularity in laboratories, by putting in some parameciums (tiny micro-predators) forcing them to try and grow bigger to avoid getting eaten. Multicellularity – its a really complex and varied story.
So what? Why do all these little creepy-crawlies matter?
Life on Earth has such a complex history behind it. First, you need to get a Universe, galaxy, star, and planet that allows for life (see our previous posts). Then you need to get life itself to occur, through complex chemical reactions in Earth’s early oceans. Then cells need to get sufficiently complex that they can repair themselves. And even then, after all that, we are still really just talking blobs. Gorgeous, fancy and complex blobs, that can move around, attack, and hunt other blobs, but still blobs. We are still so far from even the humble worm. These earliest microbes and creatures represent the history of life on our planet itself. It’s taken a long time to get here. We should appreciate it all a lot more.
