Digits digits (Life Sciences)

[sheena]

Darwinian evolution has been used to explain the diversity of species. Living in different ecological niches promotes the development of 'adapted' characteristics which enhance the organism's ability to compete, survive and breed. We have big animals living in big open spaces and small animals living in small spaces and so on. However, let's not ignore the similarities among animals - why do the vast majority of animals have five digits - not two or three or ten or twelve - on each hand/toe? If one digit is good then two should be better and ten or twelve might be better again. Why five?

[Wireloop]

[Mick Harper]

Sheena, you have to understand that Wireloop is invariably right but it takes several tries to squeeze out of his brain anything that is useful. Him and his brain being more than usually distant from one another.

Which reminds me, I am preparing a short discourse on our relationships with our brains for the TV series, so I will post up something when I've finished working out what it is.

[Martin]

Luck. They all share an ancestor that had 5 digits. This ancestor came to dominate the land and so its descendants all have 5 digits. The changing lifestyles of these descendants has resulted in changes to the morphology of the digits.

[Mick Harper]

This is extremely unlikely. Given the choice on offer, unless there was something particularly useful about five - or perhaps something that makes five the default position - we would have a much livelier array.

[Martin]

The genes which determine digit number also controls hand, feet and genital development. Mutations varying digit number would offer little advantage and most probably cause considerable harm. Some animals have adapted vestigial thumbs etc. in situations where they do not need five digits.

[Mick Harper]

Interesting but not illuminating. If you're correct (and by the way, do they really understand genetics to this degree of certainty?) the system seems capable of delivering one penis and two balls without any difficulty alongside five fingers and toes. On the other hand, as you say, the system seems in some way too basic since for example the horse has to go to all that trouble to develop one toe into a hoof and vestigialise the rest.

But even so you haven't said why it chose five in the first place. Does this number turn up elsewhere in nature, for instance in leaf patterns?

[Martin]

I think they are called hox genes. There were some women who had hox mutations they had strange hands and feet plus no uterus. A bum deal.
Some very early land animals had eight digits, for example Acanthostega, however the five digits won out and they disappeared.
It does occur a lot in nature.

[Wireloop]

[Wireloop]

[Mick Harper]

Not sure how relevant it is but those of you who read my Dance of the Great Powers will remember that
a) there are always precisely five great powers at any one time and
b) I didn't know why.
This seems a good a time and place as any to try to concentrate my thinking on this question.

My first thought was that five represented 2 plus 2 with one over as the 'balancing item' ie the fifth Great Power acted as the stabilising item. But this must be wrong because 3 would make a system too (or any odd number, come to that). Then there was the idea that Great Powers are always in the Temperate zone of the earth and therefore tend to be 'strung along' in two rows of three and two.This makes a pentagram (of sorts) but again I couldn't see why this should be so important. After that I kinda gave up and decided, "What the hell, what works works." Perhaps Mother Nature came to the same conclusion ie five is a default number, the lowest one that doesn't suffer from built-in problems. So what's so wrong about having eight digits that leads to extinction?

[Ishmael]

[Hatty]

There was a discussion on the radio about symmetry where one of the interviewees stated that the brain is structured in such a way as to recognise symmetry and she cited the bumble bee being attracted to flowers because of their symmetrical pattern (apparently bees lack a sense of direction and are colour blind). I don't know if this is accepted in the science community. Why not the other way round, that flowers evolved a symmetrical pattern to attract the bees?

[DPCrisp]

What were the bees doing before flowers?

[AJMorton]

They were being pre-bee, non-flower loving insects, I reckon.