Proconsul africanus (Hopwood 1933a, b; 18-14 mya) was preceded by Aegyptopithecus and succeeded by Ardipithecus.
Proconsul (aka Dryopithecus) looked, and probably acted, very much like a chimpanzee. It had a flatter face, a larger size and probably a shorter, stiffer lower back. Proconsul lacked a tail. The skull was round and smooth on top, like a monkey's skull.
The hands were similar to those of a human. They had not yet evolved into the long-fingered, short-thumbed hooks of living apes. So rather than walking on its knuckles, Proconsul likely continued to walk on its palms.
A new form of locomotion was invented at this time, brachiation. Hanging by the hands from overhead branches was enabled by looser arm joints. Apes can brachiate below branches that are far too thin to walk upon, typically in order to reach fruit that would otherwise be too far out of reach. Brachiators have a wide chest, not a deep one. A smaller spinal ridge makes it more comfortable for apes to sit against tree trunks and in chairs.
If you want to read the book, "From the Beginning - The Story of Human Evolution" by David Peters (Little, Brown 1991), which is where the above images were first published, click here for the PDF.