We don’t know who was the first human to take birth on this earth. For a human to take birth, it would require two other human beings. It is a classic chicken and egg problem.
Scientifically humans didn't take birth; they evolved from a single cellular amoeba found in deep oceans. As we know, monkeys and chimpanzees were our ancestors. We have evolved from them. 
Either mammals, birds or reptiles in any category of living creatures the birth is always given by females. Female birds lay eggs. Female reptiles lay eggs. A female mammal keeps her baby in the womb and gives birth to the young one. So is with humans.
A female human body too lays eggs in her ovary duct every month but the eggs cannot get fertilized on their own. They need an external source i.e sperm of a male. When it enters into the ovary and collaborates with the egg, it forms a fetus which after 9 months grows into a complete baby in the mother's womb. 
If the eggs are not fertilized, a layer of blood breaks down and comes out of the body every month and lasts for minimum 3 days and maximum 7 days. This is called the menstruation cycle. 
When a girl attains puberty at the age of 13, that means now she is biologically eligible for giving birth to a baby, she has started to lay eggs in her body. Her menstruation cycle has begun. 
Blood comes out of the body for 4-5 days on an average, and that's uncertain so a girl has to use sanitary pads and napkins to control that blood. If she doesn't use that, controlling the flow becomes difficult and it seems to be very unhygienic. 
Nowadays sanitary pads are available in the market, which gives comfort and hygiene to the girls. In early times no such pads were available and hence those 4 days were hard to pass. Women have to remain seated most of the time. Especially in the context of India. 
Even today in many villages of India, these pads are unavailable. Ladies and girls aren't aware of using these products as well as their hygiene. Many NGOs in India are working to spread awareness among the rural areas. They distribute pads free of cost, and teach them how to use it. 
In ancient times this cycle was considered as a subject of shame and something which is very impure and dirty. A girl who was in this cycle of period, was given a corner of the house to sit. She was given a separate mat to sleep on, a plate to eat, and a glass of water to drink, which nobody would touch. She cannot sit beside anyone in the house. Entering into kitchens and temples was also restricted. It was believed that if a girl who is on her periods touched anything, that thing would become impure. 
Looks like no matter how educated we may be, we continue to surrender to the ancient system and culture especially when it is related to religion.
The people of earlier times were uneducated and unfamiliar with this biological concept and hence they would have assumed according to their level of understanding. 
People in India are never ready to accept the change, except very few. People don't want to change according to time's demand. Even if they are enlightened with the knowledge they cannot leave those old rules and rituals which were created because of lack of knowledge. 
Even today except a few families all follow the same thinking which is mentioned above. The sad thing is that this is despite being educated, familiar with the reality of this process, well known about the biological process behind this. These people are more illiterate than those who were in earlier times. They are educated but uneducated fools. 
For some girls this is something to be shameful of. If accidently she faces leakage and stain it becomes something shameful for them. She feels insecure because people around them make her feel so. 
People here need to understand that it's not an easy task to give birth to any woman. From the age of 12-55 she has to face the pain she faces during this menstruation cycle, and above that feel isolated from family members and insecure among society. After this when she carries a baby in her womb for 9 months the problem she has to face is out of our imagination. The pain she experiences while delivering a baby is the most painful experience one can bear. It needs so much courage to face that, and a woman faces it, which is really appreciable. 
Menstruation is not a subject of shame. It's the most necessary biological cycle without which no reproduction is possible. The people on this earth are partial but this universe has always been unbiased. It has made the sperm of men equal to the egg of women. Both are useless without each other, both need each other for reproduction.