Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

The question of complexity is classic in the very first lectures of evolutionary biology where the teacher usually tries to tell the students that complexity does not necessary increases and that humans are not more complex than other organisms.

My questions are:

  • Why does complexity tend to increase through evolutionnary times?
    • What are the different hypotheses to explain this pattern?

When writing "Mass extinction" on google image, we find many graphs displaying the number of families (or other taxa) through evolutionary times with the 5 Mass exctinctions. What would it look like to draw such graph replacing the family richness in the y-axis by :

  • Mean complexity among all living things?
  • Complexity of the most complex taxon?

I supose that, one who wishes to answer to this post will necessarily needs to define the words "complexity". He/she might define it in terms of number of genes, number of metabolic pathway, length of the DNA sequence, number of cell types, some kind of index taken from information theory, ...When asking my questions I had in mind a definition close to "number of genes" or "number of metabolic pathways".

share|improve this question
up vote 1 down vote accepted

This is actually a very interesting yet difficult question to give a single precise answer to. I will try and summarize for you a "meta answer":

Complexity Science Some consider complexity not to be a Biological topic as such, since it is a property that accumulates in non-biological systems e.g. economics, technology, music, language - in fact anything that "evolves" through time. This new field of science is called "Complexity Science" or "Complex Systems" and is primarily a field of mathematics or information theory: Complex systems

Complexity in Biology What I can say is that these kind of questions have started an almost new field in Biology, called "diversity evolution", here is a very interesting paper: Diversity Evolution.

Defining Complexity You were right to mention Complexity first needs t be defined, and as an early field of science this is where much focus has been recently. There are quite a few definitions but it is perhaps too controversial to list any particulars... HERE is a whole UCL lecture dedicated to defining Complexity!

Topics of Complexity Science I have written enough, so perhaps this lovely diagram of the topics of Complexity Science will be of some help:

enter image description here

share|improve this answer
Thanks for your answer. From your answer I understand how broad is the science of complexity and I understand that the tendency for complexity to increase does not exist only in biological evolution but also in the evolution of music, technology, economics and other. It is very interesting. It does not answer to why complexity tends to increase (in the biological system) and how it evolved through the past time. I realize that the question might be a hard one! I'll wait to see if someone have some hypotheses for this increase and for past patterns. – Remi.b Dec 7 '13 at 16:30
"economics, technology, music, language" are all biologic, since they exist inside biological organisms. If you want to talk about complexity outside of biological realm, you can consider physics, chemistry, minerals, crystals, etc. – Rodrigo Feb 1 '14 at 8:29

Biology arises when some molecules begin copying themselves with a certain amount of error: not too much error, or there would be no heritability; but SOME error, or else there would be no evolution at all. Copying, copying, a few molecules become a multitude, and evolution spreads in all directions, to the more and to the less complex. But when a new level of complexity arises, it usually allows a new size, and a bigger organism can, for instance, eat a smaller organism. Once this level is achieved, it tends to be mantained. Again some copies will lead to smaller lineages, but the big ones have founded a niche that will not disappear by itself (it may take an asteroid). So, looking up close, evolution leads to diversity, not only to higher complexity. But since small complexity already existed, and bigger complexity is something new, we tend to perceive evolution as creating complexity - which is not a false point of view.

share|improve this answer
I think you are conflating complexity with a necessary increase in size and with speciation, which wasn't the point of the question. – kmm Feb 1 '14 at 23:47
The question was "why and how does complexity increase"? I explained it. Size and speciation are part of the answer. We needed eucariotic cells to get bigger cells. Bigger cells were mantained because they lived in a new niche, where competition couldn't expel them. We needed three germinative layers to have bigger and more complex animals. Size and complexity are like the egg and the chicken, they generate each other. – Rodrigo Feb 2 '14 at 0:14

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.