Why should you risk your own life to save another human being? Your genes are the most precious things in the whole world and you must protect them at any cost.
Jumping into a pool to save a drowning swimmer, or pulling a road accident victim out of a burning car just isn't a risk worth taking.
That person is your rival - for food, for resources, maybe even for a mate. Helping them should be the last thing on your mind.
Most human beings simply don't think like this. Yet this is how many people believe evolution explains human nature. If the twin goals of survival and reproduction are our reason for being, then we must all be selfish under the skin.
Yet every day, people across the world demonstrate their capacity to be moral, to be just. And far from being the by-products of civilisation, the instinct to put others first may be as basic as our urges to compete, to survive and to replicate our genes.
You scratch my back...
There is a rare form of co-operation in nature, practised by only a handful of animals, which could lie at the roots of our instinct to put others first.
Reciprocal altruism is a form of sharing or kindness that results in mutual benefit. To exist, it requires other animals to return the favour regularly and for an animal to be able to grant a large benefit to another at a small cost to itself.
Please give blood
One species that demonstrates reciprocal altruism is the vampire bat. These nocturnal mammals feed on the blood of larger animals while they are sleeping. But food is relatively scarce and bats regularly return home to their roosts hungry. If a bat goes more than 48 hours without blood, it will begin to starve.
If this happens, other bats will regurgitate blood into its mouth until it is nursed back to health. For this system to work, bats that have received blood must return the favour when the roles are reversed.
"These animals seem very capable of keeping track of associations over long periods of time," says Gerald Wilkinson, a zoologist at the University of Maryland. Wilkinson has also shown that bats will not share blood easily with new members of their group, suggesting that these blood-sharing associations are built up over time.
This suggests that bats may be able to keep track of their blood donations. The most obvious benefit of this skill would be to detect and recognise cheats, in order to make sure that they are denied blood in future.
Wilkinson suggests that blood-sharing between vampire bats may owe its origins to the extinction at the end of the last ice age of several important species of North American mammal.
The disappearance of the horse, camel and giant sloth from the continent would have drastically reduced the food supply for vampire bats. These conditions may have favoured the survival of bats that shared.
It's just possible that a similarly monumental period of environmental upheaval was responsible for the emergence of human altruism.
I can see for miles
Around two million years ago, climatic change caused widespread deforestation in central and eastern Africa, creating a wide belt of open Savannah where once there were trees as far as the eye could see.
The primates that lived in the forests now had to adapt to living on hostile open plains, where they would have been easy prey for formidable predators - including 20 species of big cat.
Forced to use increasingly sophisticated hunting methods on these dangerous grasslands, it is thought that our early ancestors must also have evolved advanced forms of co-operation and sharing, especially of food. But do these behaviours really set us apart from our ape cousins?
Meat sharing is also seen in non-human primates such as chimpanzees, capuchin monkeys and baboons.
But Professor Craig Stanford of the Jane Goodall Research Center in Southern California is sceptical that sharing in animals can ever be attributed to true altruism.
Greed is good
"In non-human species, any act attributed to altruism is better interpreted as selfish behaviour with altruistic by-products," Stanford explains.
There is some suggestion that individual primates will share with another member of their group to manipulate them into co-operating in future - a kind of bribe.
"Even in human societies we can still trace many behaviours seemingly carried out for the benefit of the group back to benefits for the individual," Stanford explains.
"Someone who jumps into a river to save a drowning man will become a hero. And heroes accrue all sorts of benefits. For example, they may become more attractive to women," he adds.
However, hunter-gatherer societies such as the !Kung of southern Africa share out meat equally for the mutual benefit of all members of the group.
And some scientists believe that the equality, or egalitarianism practised by many hunting groups represents a yawning gap between human behaviour and that of our simian relatives.
Professor Andrew Whiten of the University of St Andrews believes that this behaviour could have been driven by an arms race between ambitious upstarts who wanted to seize control of a group and 'counter-dominants,' those who opposed dominance of a group by one individual.
Counter-dominants might have co-operated to put down upstarts by ignoring, ridiculing or ostracizing them. In turn, the upstarts would have developed ever more subtle tactics to wrest control of the community.
This battle of wits might eventually have reached a level of sophistication where it was no longer practical for one person to dominate a group.
Whiten and others believe that this egalitarianism marked the emergence of a new form of behaviour not seen elsewhere in the animal world.
"Hunter-gatherer food-sharing appears a quite distinct evolutionary innovation," says Whiten.
And despite his general scepticism about the nature of altruism, Stanford believes a handful of human behaviours cannot be easily explained by self-interest.
"I'm open to the possibility of egalitarianism in hunter-gatherer societies. And in western societies for example, when people give blood, there's no general payback," says Stanford.
Even if human morality arose from selfish strategies to exchange benefits and solicit future rewards, we might already have the ability to transcend these urges.
Our genes have their own selfish agendas, but our minds may just be complex and independent enough to seek satisfaction of their own.