Science Questions

Why haven't crocodiles changed?

Sun, 9th May 2010

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Mike, London asked:

My question is about the stability of change with crocodiles. We learned from various sources that all birds, incredible varieties of birds, came from a small pool of dinosaurs and mammals apparently all came from a shrew-like creature. I just wondered why crocodiles seem to be very stable over the same sort of period. Is there something special about their DNA or is it just the environmental factors that keep them moulded to the way that they are? Anyway, hope all is well. Thanks very much.



We put this to Michael Benton, Professor of Vertebrate Palaeontology at the University of Bristol:

Michael -   Crocodiles are extraordinary animals.  They are not very common.  Thereís only about 15 species of them on the earth today and they havenít really changed very much for the last 200 million years, or at least to our eyes, they havenít.

Nile CrocodilesThere are two reasons that people sometimes give to try and explain this phenomenon.  We call them living fossils, meaning, animals or plants that havenít apparently changed very much.  Itís not a very precise term so we have to be very cautious about it.  It may just be our perception - they look the same to us.  But those explanations are either that they're hugely successful on the one hand or on the other hand, that they're just doing something that nothing else wants to do - so a kind of strongly positive and a somewhat negative interpretation.

So, the highly successful argument is that crocodiles are doing something remarkably well.  They're preying on animals, fish and land animals in fresh water generally, sometimes in salt water.  They're feeding in a particularly beastly manner and nothing else can compete, and they do it so well that thereís no reason for them to change their mode of behaviour.

The other explanation is that they're doing something so obscure that perhaps no other animal is interested or has had any evolutionary pressure that it should evolve into this particular niche, and therefore, nothing is really competing with crocodiles, so they can just potter on doing what they've been doing for the last 200 million years, and nothing in evolution is driving them to change.


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Well, the short answer is, after this length of time, it clearly works, so why evolve?

As recent (in our lifetime) history shows - once Australia banned the killing of crocodiles - which had become nearly extinct - they have proliferated to now dangerous proportions again, such that they are considering a cull of them.  But clearly, left to their own devices, the crocodilians numbers will expand to fill the space available to them.

I do wonder, however, whether they are somehow immune to the not quite exact duplication of cells in their DNA which effects most other species leading to mutations which eventually become new species.

nsbuk001, Thu, 6th May 2010

They have undergone some changes, e.g. down-sized ...

NB: just because the external form changed little it does not mean evolution has stopped:
e.g. it's internal organs (e.g. brain) and immune-system will have changed over time. RD, Fri, 7th May 2010

Well, monkeys still exist and we supposedly evolved from monkeys, so why did we evolve? What pressure was on us that was not on apes or chimps or crocs? Why did we suddenly develop sophisticated brains in a matter of a few hundred thousand years, yet crocs have not increased brain size in 200 million? Why did we suddenly walk upright, and chimps (which have 99% the same DNA as we do) did not. Yet we were apparently living in the exact same environment, with the exact same pressures. Clearly something HUGE is missing in the theory of evolution. It has plenty of evidence, but that evidence doesn't make any logical sense. And if croc are SO well adapted to their niche? why are there so few of them relatively speaking. If they have no competition, they should be absolutely dominating the environment, yet, what we see is actually somewhat a struggle to survive. And this is NOT due to recent human encroachment, but just to the habitat they live in, and the cycles and availability of their food source throughout the ages. Steve, Wed, 7th Oct 2015

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