Currently writing a scientific hypothesis: The Evolution of Breasts for Thermoregulation.
Some interesting facts to consider:
1) Homo sapiens are the only species where females have permanent breasts enlargements, beginning at puberty. Other species, i.e. our closest relative the chimpanzee, only develop breast enlargements during pregnancy. After lactation, the chimpanzee’s breasts will regain normal flatness.
2) Homo sapiens exhibit the highest order of paternal care from males. As a result, our species also exhibits the heaviest form of female-female competition.
From this standpoint, one might think that breasts evolved as a result of sexual selection in our species’ primitive hunter-gatherer days. However, I am arguing that breasts did not evolve because of sexual selection, but as a direct need for females to maintain core body temperature (since breasts are made of the same adipose tissue which insulates us on other parts of our body - some more than others), which is vital for embryo development during reproduction.