5 fingers

Why do we have five fingers? (and five toes)

 

green iguana (Iguana iguana)

Wild green iguana (Iguana iguana)

A straightforward answer to this question would be because we inherited this pattern of development from our early tetrapod (four-limbed land vertebrates) ancestors, exactly in the same way as the green iguana on the photo (you might click on it to enlarge).

Even though first fossil vertebrates that got out of water in the Devonian Period show polydactilia (more than five digits in manus and pes), the five digit pattern became fixed later on during the Lower Carboniferous Period approximately 340 million years ago. Since then it is very unusual to find species with more than five digits.

 

manatee (Trichechus manatus)

West Indian manatee (Trichechus manatus) is an extant sirenian.

 

 

Some aquatic vertebrates (cetaceans, sirenians, and marine carnivores) show a trend towards polyphalangy, a proliferation in the number of phalanges but not in the digits.

 

 

 

Berkeley Evolution Edu

Comparative hands of human and panda

 

We have no truly six-digit limbs in today’s fauna. In pandas’ thumbs or moles’ paws strangely re-modeled wrist bones serve as sixth digits and represent rather odd solutions to the apparently straightforward task of growing an extra finger.

 

Evolution of the horses legs

Evolution of the horses legs

 

However, the contrary (reduction in the number of digits) is quite common and many groups as ungulates, some terrestrial carnivores and birds show a trend toward fusion or loss of toes and reduction in the number of phalanges. This trait reflects the developmental-evolutionary rule that it is easier to lose something than it is to regain it.

 

Salamandra salamandra

A present amphibian, the fire salamander (Salamandra salamandra)

 

For other reasons (maybe independently achieved from the same polydactylous ancestors) most extant amphibians present four toes in the forelimb and five in the hindlimb.

 

 

 

The previous discussion would explain the proximate or immediate causes of why we have five digits in hands and feet, but if as in many other biological questions, we want to look for the ultimate cause, the answer is not so evident. No one is sure why five digits instead of say three, four, six, or even seven became the condition fixed in the common ancestor and derived to modern tetrapods (like us). There seems to be no evolutionary explanation of why the five digit condition is biomechanically better than other.

 

©Text and photos Pedro Bigeriego for WildWorldVisualas

PredPrey

Predator-Prey Equilibrium or the Life-Dinner Principle

I had always thought that for prey and predator species to coexist there must be some kind of equilibrium where adaptations in one species calls forth counter-adaptations in the other.

This process occurring reciprocally for long periods of time would result in the evolution of all the amazingly varied hunting and scape strategies and complex behaviour that we see in predators and prey today. The so-called Evolutionary Arms Race would have reached an equilibrium for both antagonists, right?

Well, not exactly.

In every single interaction event, selection pressure is much higher for the prey, because if it fails it pays with its life.

The rabbit runs faster than the fox, because the rabbit is running for his life while the fox is only running for his dinner’ (Dawkins 1979).

This Life-Dinner Principle would explain the asymmetry in the arms race and why it is really the prey that leads it and “shapes” the bodies and strategies of their predators.

A nice concept that made me quite happy when I first read it!

Pedro Bigeriego

 

Rana arbórea en PN Yasuní Ecuador

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