We can look for clues to this next crucial step in our evolutionary story in Australia.
Not in fossils, but in the bodies of two highly unusual creatures that live here.
The first is the platypus, which uses its rubbery beak like a radar transmitter to hunt for shrimp or molluscs underwater.
And the second is the echidna, which forages for ants and termites on land.
The platypus and echidna are the only two survivors of a group of mammals called the monotremes.
Trace their genetic line back, and we discover they split from all other mammals around 200 million years ago.
Because they retain traits from that distant time, they give us a remarkable insight into very early mammals
like Hadrocodium.
The most extraordinary feature of all is one that no other modern mammal has retained.
They lay eggs.
This echidna egg is tiny, only about the size of a marble.
The hatching process itself has only rarely been captured on film.
These are newly-hatched platypus young, filmed in their mother's burrow.
They are only about the size of jelly beans.
The early mammals must have laid eggs in the same way, and they inherited this trait from their reptile ancestors.
This is a view inside a reptile egg.
The embryo feeds on a supply of highly nutritious yolk.
By the time reptiles hatch, they are sufficiently well-developed to go looking for their own food.
But the platypus and echidna are very different.
Their smaller eggs contain only a small amount of yolk, so their young hatch in a far less-developed state.
They need a lot more nourishment if they're going to grow and survive.
But at Healesville Sanctuary near Melbourne, we can find delightful evidence that platypus young do develop
with great success without having to leave their mother's burrow.
Four months after it hatched, a youngster is emerging for the first time.
It has grown from a tiny hatchling to near adult size.
And that is thanks to an amazing form of nourishment that is a defining feature of all mammals.
Milk.
This rich mixture of proteins, fats, carbohydrates and minerals oozes from the bellies of female platypus and echidna rather like sweat, and provides their young with everything they need to grow.

The bilbies have also come out with the fall of night.
Few animals are as strange-looking as these inhabitants of the Australian desert.
Their large ears and long snouts are what give them their extremely developed sense of hearing and
smell.
In the dark of the night and inside the large galleries which the bilbies dig, seeing is not important and
so they are relatively short sighted animals.
It is their sense of smell which enables them to find the larvae insects seeds and fungi on which they feed and the ears are able to detect the arrival of possible enemies.
Because the inhospitable desert is also home to a number of hunters.

The hare wallabies may double use of the bush forests.
Not only did they eat the flowers fruits and shoots but they also found here a safe refuge in which
to live.
This spectacled hare wallaby is marking its territory in the full light of day.
Because once the enormous lizards of Gondwana had disappeared that daylight hours were free from these colossal competitors and some marsupials became diurnal.

There are great mountain ranges, which in winter are crested with snow.
But the mammalian characteristics of warm blood and insulating fur enables the marsupials to cope with almost anything.
The wombat has fur so thick that it can remain active throughout the winter, even in the coldest parts of Australia.
It feeds on grass and other plants, and the strong front limbs with which it digs itself burrows are equally good at clearing away snow to find food.
Its pouch opens backwards, so the youngster doesn't get a faceful of snow as mum digs in serch of meal.
Numbats live in woodland, but even there it can get quite cold at night and this family are warming themselves in the early morning sun.
Fur needs to be kept in prime condition if it's to function properly as an insulator, so grooming is essential.
These dry eucalyptus forests may look unpromising as a source of food, but there are plenty of termites.
Numbats have just the right equipment to collect them.
That spectacular tongue has to be kept well-anointed with sticky saliva, and numbats spend some time
making quite sure that it is.
With gear like that, a numbat can collect 20,000 termites in a day.

Next up are possums.
Brush tail possums are found throughout Australia, but are larger in Tasmania.
They are arboreal, usually living in tree hollows.
But a cozy woodpile is just as good.
And with a choice of exotic plants in the garden they won't go hungry.

This joey looks like it's about 2 grams, so about 16 days old.
So, 16 days ago, this young would have been born.
All marsupial young are born very immature, so its ears are folded and the eyes are closed.
Instead of being enclosed in an egg when leaving its mother like a baby echidna, this joey emerged directly from its mother's birth canal just 30 days after conception.
Its front legs are more developed and strong enough for it to pull itself up through the fur, and wriggle inside a feature that is unique to marsupials, a pouch.
Here, there is a highly developed milk delivery system.
The milk is channelled through long, fleshy tubes, teats.
A wallaby mother has four of them, and can even feed young of different ages at the same time.
She might have a young, just newly born, attached to one teat, and she'll have a young with its head in the pouch feeding from another teat.
And those two teats will be producing a milk that is of different consistency.
So, one will be to nourish a new-born young and the other's to nourish a young that's almost ready to wean.
It's a great system.
The long teats also give the young a way to cling onto their mother as she moves around.
This opossum is a marsupial that lives in South America and it has no pouch.
Its young seal their mouths so tightly round the teats, they stay firmly attached.
This may well be how the early marsupials, like Sinodelphys, carried their young around.
They were now no longer tied to a nest or a burrow like the egg-laying mammals.
But this method had one obvious drawback.
Outside their mother's body, the newborn young were vulnerable to accident and exposed to disease.