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I happened upon a book (not about biology) containing this text:

A life-changing birth occurred on the planet about 190 million years ago [...] the first tiny shrew like creature brought forth an infant and began suckling her young. [...] this furry little mammal embodied a huge leap in biological evolution

The description is nicely dramatic, but it doesn't ring true for me. While I know that evolution likely wasn't as gradual as high school books represent it, the thought that a non-mammal bore a daughter with fully functional mammary glands who actively suckled her young is a bit too much for me to swallow. For other body systems, I have read stories how they emerged gradually, for example the oldest species having a few nerve cells, then new ones having a whole nervous "network" with a few ganglions, before actual brains emerged. Or similarly, worms having a patch of photosensitive cells on the skin evolved into animals with eyes. I know that platypuses have mammary glands without nipples, but presumably even they are already much more sophisticated than the first mammary glands.

So what are proto-mammary glands thought to have been like? Did they evolve de novo especially for feeding babies, or were they maybe repurposed sweat glands or something like that? What are they supposed to have functioned like?

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the production of liquids to nourish offspring has evolved separately in several lineages, mammals have simply taken it further than any other group. it is pretty easy to see how it could evolve, any way of providing even trace nutrients or nourishment to young will have an advantage in organisms that are already engaged in child rearing. it means exposing the young to predators less often.

Mammary glands are modified sweat glands.

Monotremes do not have nipples what they have is a cluster of mammary glands on a patch of skin. the young have to suck the milk of the a cluster of hair.

The exact sequence of how lactation evolved is not well known since lactation leaves no fossil evidence.


Sources:

Lefèvre, C.M., Sharp, J.A. and Nicholas, K.R., 2010. Evolution of lactation: ancient origin and extreme adaptations of the lactation system. Annual review of genomics and human genetics, 11, pp.219-238.

Capuco, A.V. and Akers, R.M., 2009. The origin and evolution of lactation. Journal of Biology, 8(4), p.37.

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