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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, called monotremes, 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.

Echidna monotremes
Echidna monotremes

However, the platypus and the 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.

Furthermore, 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.

Monotremes

The view inside a reptile egg shows that 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 platypus and echidna are very different. As monotremes, 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.

Platypus and Echidna

It’s likely that early mammals like Hadrocodium nourished their young in the same way. First with a reduced amount of yolk, and then with milk. So, what could explain this hugely significant step?

New genetic analysis is providing the answer. Dr Henrik Kaessmann has been using platypus to investigate the DNA of early mammals.

Platypus monotremes
Platypus monotremes

Dr Henrik Kaessmann “The platypus is really an amazing creature. It’s really this crossover of a mammal and a reptile, right. And so it has a key position in the evolutionary analysis of all mammals.”

First, he looked at the reduction in egg yolk. Reptiles have at least three genes that together manufacture their large yolk. Dr Kaessmann has found that the platypus DNA records a dramatic change taking place in the early mammals.

Dr Henrik Kaessmann

According to Dr Kaessmann “We found only one egg yolk gene in the platypus genome that really was functional and was producing the egg yolk protein.”

Presumably the fact that there was only one gene which was producing yolk accounts for the fact that the platypus egg is so small?

Dr Kaessmann “Exactly.”

Dr Henrik Kaessmann
Dr Henrik Kaessmann

The early mammals must have started to switch off their yolk genes. And Dr Kaessman has made a second key discovery. The trigger for this shutdown was the arrival of the genes that produce milk.

Dr Kaessmann “So, you have the milk genes appearing that then allow for the subsequent loss of the egg yolk genes.”

But, the mammals began to favour milk over egg yolk as a way to nourish their young. And that is because milk has one key advantage. It’s on tap, and that means that none of it need go to waste. And there is no limit on how much and for how long a mother can feed her young.

And warm bodies, powerful senses, and now, milk, had allowed the early mammals like Hadrocodium to gain a foothold while the reptiles still ruled. But combining egg-laying with milk-feeding brought a new challenge. A mammal mother could not leave the eggs to hatch by themselves as most reptiles do today. She had to stay with them.

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Monotremes: Platypus and Echidna from Australia