It’s an interesting question because humanity is the ONLY existing species where the females have breasts.

It’s not clear to what extent other now extinct members of the hominid line (Neanderthals, homo erectus etc.) also had breasts, but we are certainly aware of no other non-hominid species that has this peculiar biological form.

There are some twenty characteristics of homo sapiens sapiens, aka us, that makes us very different to other “lesser” creatures. The religious idea that we are a special creation of God / the gods, may lack support in evolutionary theory and the fossil record, but is actually surprisingly well supported by an examination of the characteristics of Human Beings when compared with ALL other creatures on Earth. This topic is covered to some extent in Chapter 5 of my book CK’s Wisdom - How To Love Yourself Properly. It’s also covered in various places on this site. You can try using Humanity, Evolution, and Religion in categories and tags to find them, as well as the links herein.

All mammals feed their young through either nipples or teats. All do so perfectly satisfactorily without requiring the additional supportive tissue behind the nipple that’s found in adult human females. Indeed, some studies have shown that in at least some cases, depending upon breast size and shape, the form of the human breast makes it less effective for succouring babies than the nipple of Chimps, Baboons, or Gorillas (our nearest biological relatives in terms of DNA similarities).

In addition it is noted that teats and nipples do not appear to be of any particular sexual interest to the males of other species, including our nearest relatives as just mentioned above.

So why do Human females have breasts, and why do Human males find them so interesting?

Although Genesis 2 is very specific about God creating woman as a partner (help-meet / helpmate) for His created special godlike creature Adam, it makes no comment about her form. Furthermore it suggests that she was cloned from the rib of Adam, but still makes no comment as to why she is therefore so dimorphically different. Why breasts Lord? We thank you for them, but … why breasts? Don’t they at least bare a mention in your creation text? Apparently not.

The answer is found precisely in the fact that males find them interesting. As Desmond Morris made clear in his best selling book The Naked Ape some fifty years ago, breasts exist as a form of sexual signalling.

Somewhere in its evolutionary development the hominid line developed from being a primate that moved around largely horizontally, at least when on the ground, using four limbs, in a similar manner to modern day Chimps, Baboons, and Gorillas, to standing erect on two legs.

As discussed elsewhere the Human female has good reason to be the sexiest creature on the planet. To this end she has numerous sexual attributes and characteristics that are uncommon / unknown in other species.

Many other species use sexual signalling. The tail of the male Peacock is probably the example that most readily springs to the mind of most readers, but we are all aware that many species have colourful plumage, hair, regions etc. Both the phallus, the nose, and particularly the butt of baboons are clear examples, in a very near human relative.

And yes - Human females do develop additional subcutaneous fat deposits at puberty to give their butts a larger and more rounded form than for males. As in Baboons, this is a clear example of rear end sexual signalling.

However, once we became a vertical creature, such rear end signalling, although useful, wasn’t as effective as it had been. Up front sexual signals were also needed. And breasts, it would seem, achieve this.

Breasts are made yet more effective by dint of the fact that we, as Desmond Morris again pointed out in his title The Naked Ape, do not have fur. Exactly why not is something of a mystery. If one of these theories regarding our relative hairlessness holds true, then the development of those buoyant elevated breasts in Human females as a form of sexual signalling makes a lot of sense.

And if anyone is still unconvinced that their sole purpose is sexual, consider also the characteristic of vasocongestion. Like the penis, the ear lobes, and the nose, the human females breasts (unlike the nipples or teats of any other species) contain tissue that undergoes vasocongestion during sexual arousal. In short, the breast, like the penis, earlobes, nose, lips and some other areas, swells due to an influx of blood when sexually stimulated (including by thoughts). Enlargement of breasts by up to 25% during sexual arousal has been measured in females. Female nipple extension during sexual arousal is also common.

It should also be noted that sexual flushing also works well as a form of sexual signalling in Human Beings precisely because we are ‘naked apes’. Sexual flushing is notably more common in females, and is particularly common on the breasts.

In any case, the reason Human females have breasts is because they need them as a form of front on vertical stance sexual signalling, and they are all part of what makes the Human female the sexiest creature on planet Earth.