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Q Why do we have five digits, not four?

The earliest four-footed animals - tetrapods - did not have five fingers or toes. Of the three we know well, Acanthostega had eight on each appendage, Ichthyostega seven and Tulerpeton six. Subsequently, about 370 million years ago, tetrapods split into two major lineages - one that eventually gave rise to modern amphibians (and a lot of extinct ones) and one that eventually gave rise to mammals, birds, dinosaurs and lizards. At this time, the number of toes on the foot stabilised at five, although some groups have subsequently lost some toes. In the hand, however, something different happened. In the line going to amphibians, the number of fingers stabilised at five. In the line going to reptiles and mammals, fingers stabilised on five. Scientists are still trying to deduce why.

Digit formation in mammals and birds is controlled by overlapping sets of genes, called hox genes. Older forms of life, such as fish, do not have hox genes. A genetic mutation must have occurred which spurred the development of fingers and toes in the line that led to tetrapods.

Q Is it true that the temperature suddenly drops before dawn?

It is true that the coldest time of day is just after dawn. During the night, the temperature steadily falls as the ground loses heat, which is not replaced by the sun's rays. Even just after the sun rises, the rate at which the ground loses heat is still greater than the rate at which the sun is heating the ground, so the coldest time of day is just after sunrise.

Q Why doesn't a rocket topple over on take-off when the supports are withdrawn?

The supports are only withdrawn when the jets at the bottom of the rocket are supporting its weight. It is a question of split-second timing. If you watch a launch count-down carefully, you will see that the rockets are ignited before zero and while the rocket is still held in place. Once the jets have fired up enough to support the rocket - which happens almost instantaneously - the supports are removed.

Q How can a bite from a small insect paralyse humans?

From insects to snakes, many creatures paralyse their prey using toxins, which interfere with the working of muscles or nerves. When you want to move, your brain sends messages to the nerve fibres that activate muscle contraction. These release the neurotransmitter acetylcholine (ACh), which binds to receptor sites on the muscle surface. Many toxins are targeted at this region. For example, venom from the deadly black widow spider causes a massive release of ACh from the nerve fibres, whether or not the brain has told the nerve to fire. This exhausts the finite supply of ACh in the nerves and a victim can no longer move muscles. Some snakes use another method: the bungarotoxin in venom binds to the ACh receptor, blocking the chemical messages to the muscles. As a result, the victim is unable to move.

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