he famous partial skeletal remains of "Lucy" are dated to about 3 million years ago. We are told 'she' was about 3'7" tall, 25-30 years of age, and she walked upright. The pelvic bone structure is much narrower than in a human female and would presumably only allow the passage of a much smaller brain case for an infant:

And this is the reconstruction:

Lucy was found in an area in Africa over 1,000 miles (over 1,700 km.) northeast from where the footprints were found, and either could be plus or minus 150,000 years from the other.

"Lucy" was a remarkably complete skeleton for something that could be as old as 3.2 MYBP. The technical name is A Afarensis as she was found in the Afar region of Ethiopia. The arms are unusually long and she had curled fingers, so the creature was probably adept at tree climbing, as well as able to walk upright (although one anthropologist has argued she was only able to keep her balance if she walked with her hip joint flexed like a chimpanzee). The jaw is V-shaped like a chimp and some teeth have ape like features.

We should be very careful when all we have to work with is bones. One problem with "Lucy" was to tell which bits belonged to her and which didn't.

Here are two examples of difficulties when working with bones only:

Can you tell which is from a 3 - 3.5 MYBP "first family" of Australopithicenes and which from a 50 MYBP lemur? (a nocturnal monkey-like creature with a pointed muzzle):

The lemur hand is on the right.

And here's a present day example: can you tell which are bear paws and which are human hands?

The bear paws are top left and lower right. Were you correct?

So let's leave the Australopithecenes as they all seem to have been apes of one kind or another. There was no advantage in walking upright with a brain case the size of that of a chimpanzee or a gorilla because any similar sized 4-footed creature could probably run you down in no time -- even chimps and monkeys can sprint faster than humans today.