Multicellular Life

The first life on Earth appeared, we think, about 4 billion years ago; and for at least the next 3 billion years, it hardly changed, remaining as simple, single-celled microbes. Then, something happened that triggered these single-celled organisms to begin to team up and turn into metazoal or multicellular life. But what was it? One theory was the amount of available oxygen. Too little would not meet the needs of these more complex life forms, holding back their appearance. So how can we find out if this was indeed the case? At the University of Southern Denmark, Daniel Mills has come up with an intriguing approach: he’s been studying some of the most primitive species of life, that were the first to split away from the last common ancestor that we all share. If these early ancestors were oxygen sensitive like we are, he points out to Chris Smith, then that’s very good evidence that it really was a lack of oxygen that held back the evolution of complex life...

Daniel - We're primarily trying to figure out the historical relationships that existed between the earliest forms of animal life and then the oxygen content of the environments that these early animals lived in. We think that earliest animals emerged around 800 million years ago or so.

Chris - And the environment that they're evolving in; what were the conditions like at that time?

Daniel - Well, that's one of the primary motivations of the study. For a while it was thought that these earliest animals required relatively more oxygen than their unicellular ancestors. But then of course you have to know okay, then how much oxygen did these really animals really need relative to the unicellular ancestors? And we don't know, and and now it's actually one of the primary motivations of this experiment that we did.

Chris - Obviously, one challenge you must face is that, unlike someone who's so a palaeontologist who is studying dinosaur remains and they have something physical to look at, you're trying to build a picture of things that were around hundreds of millions of years ago; there's nothing left of them except perhaps a genetic legacy. So is that the direction you've taken, you're looking at the way in which their genes are or aren't present in what's around today to try  and work out what they must have been like, at least genetically speaking?

Daniel - That's exactly right. So we're particularly interested in how animals respond to changes in environmental oxygen availability. Previous studies have identified this pathway called the hypoxia inducible factor pathway, and hypoxia inducible factors are proteins that just respond to oxygen. And this pathway, essentially, initiates genetic changes that the animal uses to deal with low oxygen; for example reduction of red blood cells or new blood vessels or even ways of producing energy without oxygen in the first place. Prior to our study, this particular mechanism of dealing with low oxygen had been identified in all animal surveyed, but these previous studies left out two vital animal groups - comb jellies and marine sponges. And these animals are thought to be some of the first animal lineages to split from our common animal ancestors, so that they're at the very base of the animal tree. No-one had ever looked at whether or not sponges and ctenophores have this oxygen response pathway. So if sponges and ctenophores have this pathway, now it suggests that the last common ancestor of all these animals also had this pathway. And it's from the common ancestor that all modern animals have inherited this mechanism. But if sponges and ctenophores don't have it then it suggests that this was not actually present in the earliest animals that evolved later on.

Chris - So in other words, if those animals do have this oxygen sensitive pathway, these very primitive animals, that would argue that it was oxygen that was holding everything back? And if they don't have that, then it argues that perhaps something else, not just oxygen, was slowing down the rate at which multicellular animals appeared on Earth in the first place?

Daniel - If sponges and ctenophores had this pathway it would at least suggest that maybe this pathway was a necessary precondition for the evolution of modern animal life as we know it. And that until they figured this out, this oxygen response pathway that animals as we know it couldn't have evolved. But then, alternatively right, if these other animal lineages don't have it, and it wasn't present in the last common ancestor of all animal life, it was not a necessary precondition for animal life as we know it.

Chris - So the 64 thousand dollar question, or perhaps one even more valuable than that: do they have that function or not?

Daniel - Sponges and ctenophores do not have this function. So we only have genetic data from ctenophores that suggests that they lack it, but with sponges now we have the genetic evidence suggesting that they lack this pathway; we have experimental evidence essentially confirming it.

Chris - How did you prove it though? How do you know that they may not have the same genes, for example, that you and I have switching on when we see a low oxygen environment, how do you know that they haven't got equivalent other genes,, which produce the same end result, but actually are a little bit different?

Daniel - So we took marine sponges and put them in a set up in the laboratory that exposed them continuously to oxygen depleted water. We noticed that they weren't changing their genetic expression. But if they had this pathway you would have expect a shift in the sorts of genes that they were turning on and off but we didn't see that response at all. There is essentially no difference between the sponges that we looked at under low oxygen compared to the high oxygen controls.