Science in a Complex World: Pond scum offers clues to life’s puzzles

Upon seeing or hearing the word evolution, one might think of a cartoon showing a parade of increasingly upright apes and humans. Alternatively, one might envision a black-and-white picture of a bearded naturalist examining the beaks of finches. Me, I think of algae…

In their own beautiful, slimy way, algae challenge some of our most basic concepts in evolution and give us clues about how complex life might have evolved.

It’s hard to write a piece that seduces people with algae, and perhaps rightly so. Pond scum and sushi wrappings lack the wonder and warmth of dinosaurs and puppies. What algae lack in public persona, however, they make up for in flexibility, stretching many traditional notions in biology.

For example, some species of algae can take on two or three completely different forms — so different, in fact, that scientists have thought they were separate species.

Compare this with people. From birth to death, we pretty much always look like humans. Moreover, these different forms of algae vary tremendously in size, with some being single cells and others being part of large, multicellular bodies. This particular aspect of algae is interesting to me because I want to know the ways multicellular life could have evolved from single-celled ancestors. Algae are the masters of gray areas, straddling the boundaries between single-celled life and multicellularity.

At this point, it might be best to provide some useful backstory.

The first life on Earth appeared billions of years ago. Its origins are a complicated and fascinating topic that intrigues and tantalizes scientists, including some of my colleagues at the Santa Fe Institute. Nonetheless, let’s skip over all of that and just start from the first bacteria on earth, maybe 3.5 billion years ago. By our modern viewpoint, we would call these simple. They were rather plain-looking, single-celled organisms unable to kick soccer balls or write pieces for The Santa Fe New Mexican.

For a long time, it was just these bacteria reproducing and occasionally evolving new forms or functions.

Along the way — maybe a billion years later — early forms of multicellularity first appeared. They looked a bit like tiny blue-green noodles and, fortunately for us, they produced oxygen in the atmosphere. These first multicellular organisms had to wait another couple billion years to enjoy the ample multicellular company they enjoy today.

Then, in a very “short” period known as the Cambrian explosion, there was a dramatic increase in the numbers and complexity of life. This eventually led to dinosaurs and puppies, ultimately culminating with us, the humans. Or so it goes.

And this is the typical view. Life was simple, then it became multicellular and then it got complex. In fact, the evolution of multicellularity is often referred to by biologists as a “major transition in evolution.” Before, single cells were the focal point of evolution, but afterward, the focal point shifted up a level to groups of cells, i.e. multicellular organisms.

The equivalent in humans might be having children. Beforehand, all that matters are the parents. Afterward, it’s all about the next generation, the children.

This view of the evolution of life on Earth is attractive because it leads from simple to complex, from low level to high level, from microscopic to monumental. In addition, any system where we come out on top is always flattering.

It is also appealing from an evolutionary standpoint because our theories and predictions fit quite neatly. Instead of focusing on the evolution of single cells, we simply “zoom out” and group a bunch of cells together and focus on their evolution. This is why we don’t think of ourselves as an evolving community of heart cells, skin cells and the other 200 or so types of cells in the human body. Instead, we consider ourselves as a whole entity, a single human.

But here’s the rub: A lot of living organisms switch between single and multicelled forms. They are not simply one or the other but somewhere in between.

This brings us back to the algae. No matter the type of algae — red, green or brown — each has examples that move freely between being single cells and multicellular bodies.

This causes problems when we try to understand basic evolutionary forces like competition. For example, if there are two types of rabbits and one type tends to have more baby rabbits, then that type of rabbit will eventually take over the population. In other words, there is a big evolutionary advantage for rabbits to, well, breed like rabbits.

Now let’s apply this to algae. Imagine a similar type of competition between two algae, where one has more of the single-cell form and the other has more of the multicellular form. Who’s going to take over the population?

It turns out the answer is not so easy and can often be surprising. Even though algae seem simple in comparison with rabbits or humans, they are much more complex in evolutionary terms. Understanding the results of competition in organisms like algae is one of the topics driving my colleagues and me at the Santa Fe Institute.

And because there are many more organisms like algae than rabbits, topics like these are particularly important if we are to ever fully understand the evolution of life on Earth.

So far, we have a pretty good idea that algae may be more complex than pond scum would lead us to believe, but algae also cause us to rethink what an organism is.

One cool feature of algae is that some can form lichens. Lichens are those gorgeous orange or green or blue-gray creatures crusted on rocks. They are found almost all over the planet and have even survived travel in space without the need of marshmallow spacesuits.

Biologically, lichens are not one type of organism but rather complex communities of multiple species. A lichen can consist of fungi and algae, or fungi and cyanobacteria (the blue-green noodles referred to earlier). A lichen also might consist of fungi, algae and cyanobacteria; or various kinds of fungi and algae and cyanobacteria. There are many possible combinations.

In fact, a lichen in South America recently was found to be a collection of more than 100 different species! Pause for a moment and compare this to us. While we may have different cell types, they all share the same DNA. Algae, instead, are just one type of lego block, each with its own type of DNA, that can build a lichen.

But here’s where it gets really interesting. Lichens are a bit of an enigma. Although they are made up of completely different organisms, they can reproduce as a whole. This is evident when looking at a rock covered with lichens: There are often many distinct areas covered by the same type of lichen.

So the question is: Is a lichen a single entity like a human, a rabbit, a puppy or a dinosaur, or is it just a community of things like an ecosystem?

Our answer seems to depend on the exact interactions between the algae and the fungus. If we believe the algae gives the fungi food and the fungi protect the algae from harsh conditions in return, then thinking of lichens as a whole entity seems very natural. This is the same as the relationship between our heart cells and skin cells, with the heart cells allowing the rest of the body access to oxygen and the skin cells protecting us from the outside world.

Our view of the system changes, however, if the algae and fungi are less caring and sharing and more wheeling, dealing and stealing. For instance, if we believe that the poor algae are being farmed for food by fascist fungi, then we may not view this as a single organism but rather as an unfortunate interaction between two different organisms. So, our views of the lichen system seem to change depending on how we understand the fungi-algae relationship. Somehow, it seems strange that the fundamental question of what an organism is might be subject to interpretation.

Strange questions like these concerning how organisms navigate multicellular living arrangements drive me and my research collaborators. We are looking to develop a formal, systematic approach to deal with these questions — one that relies on mathematical relationships between populations.

It is our goal to try to understand how the complexity of life arose and filled the space between the boundaries of single-celled and multicelled life. And here, at least for me, the algae are a guide. Our imaginations are never as creative as life itself. My hope is that our approach can allow us to better understand the evolution of not only multicellular organisms but those beautiful, slimy beings in the gray areas, the algae.

 

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