1 Natural groups

Darwin made extensive observations on a great many creatures, including mammals, and noticed that species fell into natural groups, e.g. lions, tigers and leopards have many similarities, and resemble cats. On the basis of his observations, he was able to place mammals in distinct groups.

His work has continued, and we now recognise that mammals have evolved from a common ancestor, and have branched into many different groups, or ‘Orders’. Figure 1 shows the different Orders of mammals.

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Figure 1

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Evolutionary tree showing the origin of the main mammalian groups, each of the different Orders of mammals, and the approximate number of species in each Order.

Figure 1

What does an evolutionary tree like this really show?

To understand what the tree shows, we need to look at the different branches on the tree, and how long ago they had their origins.

It shows that Monotremes are on a separate branch of the tree, and that they branched off from the branch which led to other mammals more than 100 million years ago.

Marsupials are on a separate branch which originated between 60 and 100 million years ago.

All other mammals fall into four main groups, shown in Figure 1 in purple, red, blue and green, and all the main Orders of mammals were already present by 60 million years ago.