How are humans different from our ancestors?

If you've ever tried to construct a family tree, you understand how difficult it can be to find information about your ancestors, especially as you dig further and further back through your family history. Imagine, then, the difficulty of piecing together a family tree that stretches back millions of years, long before Homo sapiens ever walked the planet.

And yet scientists and archaeologists have learned a remarkable amount about our ancestors from fossils and geological clues unearthed since Darwin developed his theory of evolution. With each discovery, we have a better understanding of how similar, and how different, our ancestors were from us.

Among the earliest of those ancestors, known generally as hominids, were members of the genus Australopithecus. Heavy-browed and covered in hair from head to toe, australopithecines bore only a faint resemblance to humans, with females typically standing between 3.5 and 4 feet (1.1 and 1.2 meters) tall and males standing up to 5 feet (1.5 meters) tall [source: UCSB]. For instance, Lucy, the famed 3.2 million-year-old skeleton discovered in Ethiopia, stood only 3.5 feet (1.1 meters) tall [source: National Geographic].

Australopithecines roamed the forests and grasslands of Africa as many as 4.4 million years ago, and just like us, they roamed on two feet [source: UCSB]. Scientists theorize that this important adaptation allowed our ancestors to forage among Africa's savannahs when apes and chimpanzees were confined to the forests. As a result, australopithecines were able to include fish, turtles and even crocodiles in their diet -- foods you probably won't find at the local restaurant, but rich in fatty acids. Whether our forerunners hunted those animals as we do or scavenged prey from other hunters isn't clear, but their meat proved an important supplement to the fruits, vegetables, roots and insects they likely ate.

According to a recent study published in the journal Proceedings of the National Academy of Sciences, that diet may help explain why, around 2 million years ago, the australopithecine brain began to expand rapidly in size. While they first had brains about a third of the size of the human brain (or no bigger than a chimpanzee's), later species display signs of having developed higher cognitive abilities [source: Stanford University].

Like humans, our ancestors appear to have developed and used tools, rudimentary as they may have been. For instance, archaeologists discovered marks on bones found near Australopithecus fossils that indicate the animals were butchered. Australopithecines likely used rocks with a sharpened edge to handle the task, but their tools were so primitive that scientists have a hard time distinguishing them from broken rocks.

Evidence also shows that, like us, our ancestors were social creatures. They appear to have constructed shelters where they would gather food and share it with each other. Some scientists hypothesize that while mothers cared for their infants, males may have helped with other tasks (a phenomenon daddies around the world now refer to as "making a grocery run") [source: UCSB].

While we still have many questions about australopithecines, each new discovery seems to confirm their place on our large and complicated family tree.