Re: got milk

[Pieter.Depuydt@rug.ac.be]

Earl Wood wrote:
> I would like to get some help from anyone who has the time or the
> inclination, my main topic is "mammogenesis". I can rationalize the
> evolution from egg to placental birth, which may be wrong, but what
> I really would like is some input on what was the mechenisum for
> jump starting the mammary system, an intirely new concept at the
> time it was initiated. all answers or posts no matter how far out
> will be appreciated as food for thought.

Fossils don't help us much here, as soft tissues very rarely 
fossilize. So how exactly mammary glands originated, remains 
speculation, but examination of present day taxa, histology and 
fysiology can offer some hints.

In origin and structure, mammary glands are very similar to sebaceous 
and certain sweat glands (these are apocrine glands: the complete 
cell content is secreted); apocrine sweat glands have a 
scent/pheromone function rather than thermoregulatory and excretion/ 
fluid regulation.
Mammals have a richly vascularized and glandular skin which is 
probably the primitive condition in amniota, since such a skin is 
also present in extant lissamphibia (frogs, salamanders, caecilia). 
There is even a very rare fossilized specimen of therapsid skin, 
found in the Early Late Permian of the Urals and attributed to the 
very primitive therapsid Estemmenosuchus: already at this early 
stage, therapsids had a soft, pliable and highly glandular skin. 
So relatively little modification was required to evolve mammary 
glands out of these already existing glandular structures: the 
secretion would contain some more proteins, lipids, minerals and 
vitamins. Nipples evolved later on (notice nipples are absent in 
Monotremata).
The origin and evolution of mammary glands must have been linked to 
the evolution of parental care. Again fossils don't tell us much, at 
best there is indirect evidence. A famous fossil from the Early 
Triassic of the South African Karoo, 'Mother and Child' shows an 
adult Thrinaxodon in close association with an immature (one-third 
size) specimen, suggesting offspring stayed with adult animals for a 
while in this (rather primitive) genus of cynodont. The evolution of 
a bony secondary palate in cynodonts and advanced ('bauriamorph') 
therocephalians is considered by some (Maier and Van Den Heever) as 
an adaptation not only linked to better food processing but also to 
suckling in neonates.
In mammals, pheromones have an important role in keeping a close 
contact between parent and newborn offspring; non-mammalian 
therapsids had deep nasal cavities with extensive attachment for 
cartilagenous choanae, probably had a vomeronasal organ and probably 
had good sense of smell; it can be easily imagined that additional 
nutritive value of the maternal secretions, initially meant to keep 
the young ones close to the mothers body, certainly was advantageous, 
in that neonates could be held longer and closer, thus safer, with 
the parents. Ultimately this could have lead to a fully nutritive 
secretion, called 'milk'.

Hope this was some 'food for thought' and discussion

Pieter Depuydt