A glance at the evolutionary family tree of humans reveals that the three earliest-known branches ae full of species that appear far more ape-like than the "caveman" stereotype associated with the Homo branch. Some of these strange-looking guys and gals are evolutionary ancestors of humans, but others might be hominids that just co-existed with the ancestors of modern humans, meaning we're related, but we didn't necessarily evolve from them.
Thanks to the efforts of talented sculptors and scientists, we can look at models of these early hominids in museums and online, but the truth is that most of what we know about these species is from a few scattered bone fragments. The fossil record has been filled in tremendously since the first findings in the 1920s, so we're learning an extraordinary amount about the history of human evolution each year. Read on for a brief tour of the major hominids that predated the branch that eventually lead to Homo sapiens (that's you!).
In 2002, a team of anthropologists made a discovery they claimed would "have the impact of a small nuclear bomb." That discovery was nine cranial specimens (i.e., skull chunks) from what might be the earliest known hominid ever discovered: Sahelanthropus tchadensis, a species that may have lived in West-Central Africa about 7 million years ago. Some experts disagree and think S. tchadensis was just an ancient ape, but the team that made the find thinks they discovered the oldest species ever found belonging to the human family tree, with human-like canine teeth and foramen magnum (the hole in the skull where the brain meets the spinal cord), which indicates that S. tchadensis may be the earliest known hominid that walked on two legs.
The second-oldest hominid is Orrorin tugenensis, discovered in 2001 in central Kenya and dating back about 6 million years. The most human-like feature of O. tugenensis was its small teeth with thick enamel, which they used to munch on mainly vegetarian fare (excluding insects). Otherwise, O. tugenensis was likely far more ape-like than human-like, except it walked on two legs, as evidenced by the discovery of an upper femur bone typical of a bipedal lifestyle. Paleoanthropologists still think that the chimpanzee-sized O. tugenensis spent a lot of time climbing trees, lending it a "novel combination" of ape and human characteristics.
Paleoanthropologist Yohannes Haile-Selassie accidentally discovered a new species, Ardipithecus kadabba, when he found a piece of jaw lying on the ground in Ethiopia in 1997. Later discoveries of hand bones, arm bones, foot bones, clavicle, teeth, and a toe helped date the species to around 5 million years old. The toe indicated that A. kadabba likely walked on two legs, and the teeth, useful for chewing fibrous nuts, separated this species from the similar, but much better-known, Ardipithecus ramidus.
The discovery of Ardipithecus ramidus in 1992 marked the earliest known hominid whose teeth indicated possible meat-eating. The roughly 4-million-year-old species - the most complete skeleton of an early hominid ever found - may have descended from Ardipithecus kadabba, an earlier species also found in Ethiopia, with teeth more suitable for hard, abrasive foods. The teeth of A. ramidus have an enamel thickness somewhere between a chimpanzee's and Homo species, meaning there were most likely omnivores. Other human-like features include diamond-shaped canines and evidence of walking on two feet during a stage of human evolution when hominid bodies were adapting more and more to living in two worlds: in the trees and on the ground.