Why does poo smell bad? A brief explanation of the process of natural selection.

Nephew: You said core happiness comes from satisfying long-term, ongoing innate needs. What did you mean?

Uncle: Do you know how natural selection works?

Nephew: Sort of.

Uncle: Do you know why poo smells so bad?

Nephew: My poos don’t smell bad. They smell like lavender.

Uncle: Faeces smell bad because we evolved to perceive them that way. If our early ancestors had found the smell and taste of faeces enjoyable, or even neutral, in times of famine they would have eaten their faeces. Or eaten someone else’s. That would be bad for their health and they may even have died. Only the ones who found the smell and taste awful would have refrained from eating their faeces, and they got to live long enough to mate and pass on their genes.

Nephew: I bet if they’d had the right spices . . .

Uncle: Camels store fat in their humps and that fat provides them with energy when there isn’t food around. Let’s say millions of years ago a camel is born with genes giving it a bigger hump than most other camels. In a severe drought that camel, having a larger energy supply on its back when food is scarce, is more likely to survive and pass on its genes. Over millions of years, with similar situations occurring, all camels end up with the gene and bigger humps. Natural selection has ‘guided’ a physical change.

Nephew: Good on it.

Uncle: When male giraffes fight each other for mates, they bash each other with their necks. The ones with bigger and more powerful necks are more likely to win the battle, get the mate, and pass on their genes.* Over millions of years this results in giraffes having bigger necks. That’s another physical change ‘guided’ by the process of natural selection.

Nephew: What if a giraffe is born with genes giving it an extra leg?

Uncle: The giraffe would have trouble running and be caught by a predator, and not live long enough to pass on its genes. Therefore, we won’t see many giraffes walking around with five legs.

Nephew: That would be cool though. Where is this leading?

Uncle: A monkey born with a propensity to break nuts and eat the kernels has an abundant food source, and is more likely to survive a drought and pass on its genes. Over time, all its descendants will have the propensity to break nuts and eat the kernels. Natural selection has ‘guided’ a behavioural change.

Nephew: So, although gene mutations are random, over generations the mutations beneficial to a species can become normal to the species?

Uncle: Yes, because the beneficial mutations increase the chance that the creature will live long enough to pass on its genes. That’s the process of natural selection. It is generally thought that most physical features and behavioural traits come about in this way. As a result, species evolve and new species are created.

Nephew: How do we get new species?

Uncle: One way is to have a population of animals divided in some way, by geography or by climate, or by time, for example.

Nephew: Like . . .?

Uncle: Let’s say on one side of a mountain range there are wide open spaces, and the giraffes regularly fight with their necks to win mates. On the other side of the mountain is jungle, and a giraffe born in that jungle with a longer neck will have no advantage. It might well injure itself swinging. So, that ‘long neck’ mutation isn’t favoured. Over tens of thousands of years, the two groups of giraffes will develop so many differences that if you were to bring them together and mate them, they could not produce fertile offspring. The two groups of giraffes would have become separate species.

Nephew: Did that happen?

Uncle: It may have. The giraffe’s closest living relative is the okapi, which lives in the Congo rainforest. Its neck is much shorter.

Nephew: Another example?

Uncle: As you go higher up a mountain the temperature changes, the plants change and the terrain changes. Over time, a population adapting to those high conditions through natural selection could become so different to their cousins down at sea level that they could no longer mate with them. Result: new species.

Nephew: What is a subspecies?

Uncle: Imagine a thousand giraffes on a large island in a river. They become isolated when the river fills with crocodiles. Over a long period of time, the island giraffes develop their own characteristics, such as different markings or shorter necks. If these island giraffes were transported to the mainland and could still successfully breed with the mainland giraffes, the two groups would be the same species. However, because of their different characteristics the island giraffes, the minority, would be labeled a subspecies.

Nephew: Simple really.

Uncle: Nature is not always that clear cut. A species can be determined in other ways, too. The examples I gave are simple examples of natural selection.

Nephew: What’s another example of how a species can be determined?

Uncle: When animals could mate and produce fertile offspring, but don’t want to. For example, some frogs mating only in the morning might share a waterhole with identical frogs that mate only in the evening. Because those two groups of frogs don’t mate, divided by time, they’re different species.

Nephew: Even though if they did mate, they could produce fertile offspring?

Uncle: Yes. And, in Africa, red cichlid fish could mate with blue cichlid fish and produce fertile offspring, but they don’t. That makes them different species.

Nephew: What makes the red fish different from the blue fish in the first place?

Uncle: It has been suggested that if cichlids are separated for a while – for example, they live and breed in separate reefs for many generations – they can, through random mutations, develop differences, like colouring. If that different colouring is enough to prevent the red and blue cichlid fish from mating when they do happen to meet, that’s enough to make them two species.**

Nephew: Would the genetic differences between the two groups of fish, or frogs, eventually widen to the point where even if they mated they would be unable to produce fertile offspring?

Uncle: That’s right.

‘Natural selection penalises mating with the wrong species, especially where the species are close enough for it to be a temptation, and close enough for hybrid offspring to survive, to consume costly parental resources, and then turn out to be sterile, like mules.’
Richard Dawkins and Yan Wong, ‘The Ancestor’s Tale’. P 389.

Nephew: From what you say about natural selection, if you go back far enough, everything is related to everything. Are you saying lizards are related to giraffes?

Uncle: Their common ancestor goes back more than a hundred million years.

‘An eel-like creature from 505 million years ago was a forerunner to all vertebrates, from fish to humans. Fossil evidence confirms that Pikaia gracelens had a rod of elastic tissue running along its back, making it the oldest chordate ever found.’ 
New Scientist, 10 March 2012

Nephew: Did we humans evolve from gorillas or chimpanzees? Or from monkeys?

Uncle: From none of those animals. We do have a common ancestor that existed more than sixty million years ago, and it probably looked something like a nimble rodent. Over a long time, over large areas, and in varying conditions, those creatures evolved into different animals, depending on the environmental forces.

Nephew: How do you mean?

Uncle: A simple and speculative example: if the rodent-like animals lived in rainforests that offered abundant food in the trees, they would probably stay in the trees, and over millions of years become monkeys or apes. If any of those creatures had been born with the inclination and ability to walk on two legs they would have gained no significant survival advantage, so that mutation would quickly be bred out. Such a population might eventually evolve into another type of ape, but it wouldn’t evolve to be a bipedal ape.

Nephew: Go on.

Uncle: Let’s say another population of the same rodent-like animal lived in savannah plains, and found food in the long grass. The ones born with a mutation allowing them to stand on two legs and see above the grass might have a significant advantage, and be more likely to survive and produce offspring. Over time  they might evolve into bipedal, land dwelling apes.

Nephew: I heard another theory: we became bipedal because being able to wade upright in water was a big advantage. They could find food in the water, and carry their infants at the same time.***

Uncle: That may be true. But you get the idea: the animal evolves in response to its environment. Different environment: different animals come into being.

Nephew: So that’s natural selection? How does evolution explain the origin of life?

Uncle: It doesn’t. It’s not meant to. The origin of life has nothing to do with evolution. It’s a different topic entirely. Don’t confuse the two.

Nephew: Do other factors contribute towards evolution?

Uncle: One factor is epigenetics. That’s when genes are influenced by the environment. For example, a creature living in drought conditions, unable to feed itself properly, might give birth to young smaller than normal. The drought might then break, and the young would grow up in much better conditions. However, when it’s their time to give birth they might also give birth to young smaller than normal.

Nephew: Why would that be?

Uncle: Had the drought not ended, then giving birth to smaller young might be an advantage for those young – they would need less food to stay alive.

Nephew: Another example?

Uncle: A creature living an abnormally stressful life might give birth to young that grow up more prone to becoming stressed than they otherwise would be. When they give birth, their young might be born with genes switched on to make them stressed in even mild circumstances.

Nephew: So that’s epigenetics? Anything else?

Uncle: Some viruses contribute towards a creature’s evolution by infecting the creature’s sperm or egg, thereby changing the creature’s DNA slightly. Those changes would be inherited by its young, and if those changes are beneficial they will be passed on to future generations. For example, primates like chimpanzees, gorillas and us have within us ancient virus DNA that helps our females give birth to healthy young.

Nephew: So okay, what does all that have to do with core happiness? No. Wait. Tell me tomorrow. I’ve heard enough. You could talk a chair into running away.

Uncle: You cheeky blight.

* Robert E Simmons and Lue Scheepers, New Scientist.

** From the book, ‘The Ancestor’s Tale’ by Richard Dawkins and Yan Wong.

*** That ‘Aquatic Ape’ theory was put forth by Alister Hardy and championd by Elain Morgan, according to Richard Dawkins and Yan Wong in their book, ‘The Ancestor’s Tale’.
  In pages 112 to 115 of their book they provide other theories: anthropologist Owen Lovejoy suggested that by standing on two legs, the apes’ hands were freed and that enabled them to do things like carry infants, or food. That ability was enough to confer upon them a significant survival advantage.     
  In Jonathan Kingdon’s book, ‘Lowly Origin’, he describes his ‘squat theory’. By being able to squat, the apes could turn over rocks or leaf litter to find for insects, worms, snails and other nutritious morsels.
  And on page 322 of ‘The Ancestor’s Tale’, Richard Dawkins and Yan Wong themselves suggest the possibility that one particular ape made a habit of standing on its hind legs and as a result enhanced its sexual attractiveness and social status. The gimmick became fashionable with others following suit, and the ones managing to maintain their posture mated often and passed on their genes.
  Susannah Thorpe and her colleagues of The University of Birmingham suggest another possibility: that our ancestors evolved to stand on two legs while still in the trees. Balancing on two feet and using their hands to hold branches for balance helped them reach the fruit on small, outlying branches. (New Scientist, 9 June 2007)
  The Ancestor’s Tale, A Pilgrimage to the Dawn of Life.
Richard Dawkins and Yan Wong.   Weidenfeld & Nicolson  2016

This entry was posted in Why does poo smell bad? - A brief explanation of the process of natural selection and tagged , , , , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s