Zinjanthropus boisei, also known as Australopithecus boisei and Paranthropus boisei, was an early tool-making hominid which lived between 1.2 and 2.6 million years ago in Africa. The first Zinjanthropus boisei specimen was discovered by Mary Leakey in the Olduvai Gorge in the 1950s.
The first boisei specimen, found by Leakey, was given the nickname Nutcracker Man and named after her team's financial backer, Charles Boise. This hominid's brain, at about 500 square centimetres, was comparable in size to those of Australopithecus afarensis (the species to which the famous "Lucy" belonged) and Australopithecus africanus, but its jaw and teeth were specially adapted for chewing - in fact, it had twice the molar surface of anatomically modern humans. This probably means it was accustomed to a diet similar to modern-day gorillas, high on nuts, roots, and seeds.
The best preserved Zinjanthropus boisei specimens all date from about 1.7 million years ago. They include Nutcracker Man (now designated OH 5) as well as two more discoveries by Richard Leakey, both in Kenya. The current thinking is that Z. boisei (now P. boisei, following its reassignment to Paranthropus) was descended from Paranthropus aethiopicus, skeletons of which have been found just a few hundred thousand years below those of P. boisei. Like all Paranthropus species, it was a member of a side branch in hominid evolution which did not lead directly to Homo sapiens. It may have coexisted with Homo habilis and Homo erectus, both of whom are closer to the human line (and may in fact be ancestors of modern humans).
The fact that Z. boisei has been named to three different genera since its discovery (Zinjanthropus, Australopithecus, and then Paranthropus) is an indication of just how unsettled our understanding of this period in hominid evolution really remains. This does not mean that many of those archaeologists convinced that humans developed through natural evolution are questioning their beliefs - but it does mean that as more and more specimens are discovered, we are gradually becoming aware of just how varied and diverse the hominid family tree really is. Identifying which species were direct ancestors of humans and which belonged to one of a number of now-extinct branches is therefore extremely difficult and in many cases largely speculative.
There are other questions about Z. boisei which have still not been answered. Whether Z. boisei actually made its own tools, for instance, is an open and frustrating question. On the one hand, stone tools have been found in the same layers (and therefore same age) as Z. boisei specimens. On the other hand, we have not yet found them together in such a way that we could clearly say the tools were being used by Z. boisei and not some other hominid with which it was sharing the African savannah. We are also not sure why it had such large jaws and teeth (the theory that it needed them for a nut-and-seed diet is still just that, a theory), nor why they went extinct after surviving for a million years, far longer than modern humans. It is possible that gradual climate change led to a fall in the food species that Z. boisei relied upon, which eventually grew so severe that the species simply died out.