In the middle layer of Hadar, the Denen Dora, fossils of water-loving antelope and grazing (grass-eating) mammals are most common. There are also lots of hippo bones. These types of mammals tell us that the Denen Dora had open, wet habitats—maybe marshes or grasses around a big lake. A good example of this habitat type can be found in the Okavango Delta in Botswana today.

The top layer of Hadar, the Kada Hadar, is full of fossils of antelope, which would have lived in grasslands, like wildebeest and gazelles. Other open-habitat mammals, like the warthog-like pig, Metridiochoerus, are common. Thus, it appears that Lucy’s species is last found in the sediment layer at Hadar that had a mixture of grasslands with some trees, sort of like the modern Serengeti in Tanzania.

Taken together, the fossils of these mammals show us that Lucy's species lived in a great variety of habitats. Australopithecus afarensis could be found in a mix of woodlands, swamps, and grasslands through time.

Taphonomy—missing bones

Three million years ago, a monkey died and fell down from a tree onto the ground. That night, hyenas came along and ate some of the carcass, removing the monkey's back legs. Then, in the next week, a stampede of wildebeest came along and trampled the remaining monkey skeleton. The thin, fragile bones of the skull were shattered, so all that was left of the skull were the teeth. During the next heavy rain, these teeth floated away from the tree and out onto the floodplain by the river. This is where they were buried and fossilized.

Three million years later, a scientist walks along a dry riverbed in Ethiopia, at Hadar, the same site where Lucy’s bones were found. She spots the glint of a small monkey tooth in the rock, and then another. Three million years after the monkey’s death, all that can be found of it are two teeth. 

The story of our poor monkey reminds us that fossils sites may not preserve all of the plants and animals that lived with early hominins. For example, fossils of rodents (like mice and rats) are often rare or completely absent from fossil sites. Pollen may or may not be preserved, so we might have to reconstruct habitats without plant remains. Why does this happen? What are the processes that determine this? The answer to these questions lies in the science of taphonomy.

Taphonomists are scientists that study the processes that affect an organism from the time it dies until it is found as a fossil. By understanding taphonomy, we can estimate "taphonomic biases." It is important to estimate taphonomic biases because they may skew our views of how common animals were on ancient landscapes. This can lead to an inaccurate habitat reconstruction.

The big picture

Knowing the abundance of different types of extinct mammals allows us to reconstruct the habitats of our earliest ancestors. However, sometimes we are interested in other questions—what exactly did fossil mammals eat? What can plant remains tell us about early human habitats? How did the evolution of new behaviors in early humans affect other mammals that lived in the same habitats?