- Only one sperm is needed to fertilize an egg
- Menstrual cycles show considerable variation both in length
- During birth, the human baby rotates in a complex manner
(CNN)The evolutionary background of human reproduction can shed much-needed light on some of life's most intimate mysteries.
In my book, I trace our reproductive lifeline back to its earliest roots, revealing the hard science -- and the primate origins -- behind sex cells, mating behavior, gestation and the way we care for our young.
My main aim is to deal with issues that are of practical importance in everyday life. In that light, here are some of the questions that I tackle.
1. Why are a man's testes located outside the body?
In most mammals, including all primates, the testes descend from their initial position next to the kidneys into a pouch-like scrotum outside the body cavity.
Descent of the testes has been explained in various ways.
The most widely accepted explanation is that descent of testes is connected with the raised body temperature that is typical of mammals. It has often been baldly stated that sperm production cannot take place at body temperature. We know, for instance, that infertility results if a man's testes remain in the body cavity. Yet sperm production clearly can take place inside the body, because some mammals -- such as dolphins, rhinoceroses and elephants -- produce sperm with testes that do not descend.
Instead, it seems that a lower temperature is better for sperm storage. Mammals with undescended testes have various special adaptations to store sperm at sites where the temperature is lower.