I have a question about the average complexity of life on Earth.  It seems like, compared to 500 million years ago, creatures have been getting more complex, with big brained mammals and all.  But at the same time, it seems like some limit has been reached and lateral movements are taking place (just as they always have), meaning there is no trend in one direction or the other.  Am I really more complex than a housecat, or a mammal from 30 or 40 million years ago?  Aren't we all built pretty much the same?  I know some kooks thing we're moving to some evolutionary critical mass, and others say we can't speak of evolution at all in these terms and that increasing complexity is an illusion.  Do we know the real answer?  What would a graph look like starting a billion years ago?  I realize that I haven't defined complexity, and maybe that's part of my problem.   But I'm just speaking from popular perception here.  How would a trained biologist look at this?

Thanks!

Well Brian, you have hit the nail on the head - complexity is very hard to define. Humans tend to become very inward when thinking about things like thisand focus on themselves. Look at the (supposedly) humble octopus.

Many can change not only the colour but also the shape and texture of their skins. They are highly intelligent animals, capable of learning quickly and exhibiting complex behavious. Their eyes are much better than ours having better acuity, no blind spot and they can see UV light. To cap it all off they have highly complex nervous systems controlling their arms and suckers - in short it is very hard to think of them as simple, yet they outdate humans by tens of millions of years, and cephalopods as a whole were around long before mammals came on the scene.

It is true that diversity appears to have increased over time, and that complexity has increased too (we are more comples than the amoebas of a billion years ago!). However, quantifying that really requires an impossible amount of very arbitrary decisions as to what 'complexity' is. What you can say (though it is a bit of a 'get out' answer) is that by definition, everything that is alive, and has ever lived, was adapted to its environment to a greater or lesser degree and survived. That is quite some achievement when you think about it.

I am not sure that increasing complexity is an illusion. With each wave of extinction, new ecological niches evolve and animals diversity to fill these in new and novel ways that will inevitably cause competition and complexity. But many simple things done well, and in the right conditions (look at sharks) will survive for a long time.