Do roosters have to be with hens for these to start laying?

Not at all. All a hen needs to start her laying cycle is to have an adequate body weight and a longer period of light. This way the hen 'thinks' that spring is near and will start to breed as there will be enough food to feed her chicks.

Why do some eggs have two yolks?

Really it is a double ovulation. The younger the hens are the higher the incidence. This is because they are still "learning" how to lay and are more prone to these kinds of "accidents".

Is it true that white hens lay white eggs and that brown hens lay brown eggs?

No. Actually this is controlled by completely different gens and there is no relation. What is true is that the white commercial strain lay more white eggs and brown hens lay more brown eggs or it may also be the opposite.

When were hens domesticated?

Hens were domesticated in China in 2000 b.c. Since then it has been difficult to find an animal that has developed such a close and prolific relationship with mankind. Hens and their eggs are mentioned in the Bible, in the books by Hippocrates about food in ancient Greece, in the De Re Coquinaria by Apicius, the great cookery book of the Roman Empire. They are part of the traditions, both in the West and East, like the Easter Egg. They also appear in fables (the hen of the golden eggs) and in popular folklore in different cultures.

Currently, there are 13,000 million domestic hens, although it is nearly impossible to know the exact number due to the great presence of hens that have not been counted in the rural world both in the first world and in developing countries. Their meat and eggs are a global source of energy and proteins as many different cultures (Christian, Muslim, Hindu…) allow their consumption. It is difficult to find a more domestic animal than a hen.

What came first: the egg or the hen?

The egg. This is because it is a reproductive system prior to hens and even to birds in general.