Filling in the twigs on our branch of the tree of life

In today's big section on evolution in the New York Times, John Noble Wilford explains the explosion of new material and new understanding of the human family tree. Through the 1990s and into this century, new fossil discoveries have pushed our understanding of hominid origins back in time, and refined our knowledge of how we got from those origins to our current form.
Those new fossils are helping molecular biologists trace the ways that our genes evolved over millions of years, and those insights are sending anthropologists and systematists back to the bone to test new hypotheses. Wilford explains:

The new finds have filled in some of the yawning gaps in the fossil record. They have doubled the record’s time span from 3.5 million back almost to 7 million years ago and more than doubled the number of earliest known hominid species. The teeth and bone fragments suggest the form — the morphology — of these ancestors that lived presumably just this side of the human-ape split.

"The amount of discord between morphology and molecules is actually not that great anymore," said Frederick E. Grine, a paleoanthropologist at the State University of New York at Stony Brook.

Six years ago, a scientist observed that "We are indeed coming very close to that point in the fossil record where we simply will not be able to distinguish ancestral hominid from ancestral [chimpanzees] … They were so anatomically similar."

Eric Delson, a scientist at the American Museum of Natural History, pointed to discoveries since then of hominids 6 and 6.7 million years old, close to the time when the human and chimpanzee lineages diverged. "These are clearly the earliest hominids we have, but we still know rather little about any of these specimens. The farther back we go toward the divergence point, the more similar specimens will look on both sides of the split."

Even now, we look much too similar to our primate cousins for some people's tastes. Queen Victoria is reported to have written that an orangutan she saw was "painfully and disagreeably human."

In 2004, I wrote the following as I looked at a 13 million year old hominoid skull, a predecessor to the common ancestor of humans and chimpanzees:

A few years back, I was skinning a baboon that had died in a zoo, and been given to the museum. Skinning animals is interesting, but rarely stunning. It involves a little cutting, a lot of gentle tugging, and great care when you finally get to the hands and feet. Mouse feet, cat feet, deer feet, all look pretty normal. But a baboon has fingerprints. They don't have claws, they have nails. There's something creepy about skinning a hand with fingerprints.

There's also something eerie about looking at the earliest known hominoid, a possible ancestor of modern humans and apes.

…

Those eye sockets, the nose, the teeth, are all evidence of the first branch on an evolutionary path distinct from monkeys and gibbons. It is one step in a long and winding path that leads to me typing this. It is also a path that lead Homo erectus to an Indonesian island, where they hunted elephants, rhinos and dragons, getting small with time, living and dying, until a volcano finished them off, right around the time that Homo sapiens showed up on the same island.

A lot has changed in 13 million years, but no one can look at that face and not recognize something familiar in it. It isn't anything as unique as a fingerprint, but it is special, and human.

Step by step, scientists are bringing together bones and genes to find out what that path looked like, and what lay down some of the roads not taken.