I find your lessons a bit hard to follow (ie, lots of information in a small package - well done), so have been supplementing by googling and rounding them out. I've never had much interest in the actual science of evolution before, so you have sparked that. Thanks. I do have a question about this lesson, just a small one.
My reading showed that 'joeys' are nursed for 350 days, and humans also breast-feed for at least a year (under normal conditions, ie, before industrialization and other factors gave women the option to refrain from breast feeding). This short term of breast feeding is a 20th century practice, and has little to do with women's ability to produce milk for years after birth.
The practice made popular in the 1950s, of feeding infants Pablum (ie, cereal grains) beginning at 3 months is now known to be a bad nutritional start. Mother's milk is all that is required, and indeed, it is recommended that 12 months be the earliest cutoff (medically) for breast feeding, even when the child's diet is already inclusive of other foods.
Are we (modern technological culture) effecting an eventual biological change, by rejecting 'nature' and turning to substitution for mother's milk? In other words, will we 'lose' the ability to lactate, as more generations of babies are not fed from the breast? In effect, 'use it or lose it'?
And I don't expect a 'defining answer' ... it just made me think of all the messed-up, unnatural things we are doing to ourselves and the planet, and left me wondering what kind of genetic changes may take place down the road. Or perhaps I have just read too much sci-fi in my life (not a bad thing, considering Verne and others who 'imagined' the future). : )