Sometimes we think of human evolution as being distinct from the evolution of other animals. I think it important to remember that it's not, and that our own evolutionary history follows the same patterns, and is shaped by the same processes, as the history of all other living things.
This is demonstrated particularly clearly by the title & content of a paper that I first read a few years ago: Adaptive radiations and dispersals in hominin evolutionary ecology (Foley, 2002).
(Just a couple of points if you are reading the original article. One is that it was published before publication of evidence suggesting that Sahelanthropus might have been bipedal, & also before the discovery of Homo floresiensis. The other is that there are probably more species mentioned than you know about – or need to know about!) The paper's worth looking at here because a) it discusses the concept that hominin history demonstrates a series of adaptive radiations, and b) it links these radiations to changes in ecology.
Foley presents a phylogeny for the hominin species known in 2002 – and reminds us that it's actually quite hard to be sure that you're dealing with a particular species when all you have is bits of the skeletal remains of an organism. He characterises an adaptive radiation event as "a dispersal or series of dispersals of a common ancestral stock…, leading to morphologically diverse forms, largely through a process of allopatry and local adaptation and drift."
Foley comments that "… at various points in the course of hominin evolution, a group of hominins was 'evolutionarily successful'. Such success is indicated by a pattern of geographical expansion, followed by some level of population divergence." Three such events involve:
1) the early australopiths, perhaps related to the fact that they were bipedal. Possible adaptive advantages for bipedalism include enhanced thermoregulation and the ability to wander (& forage) further in an open environment with less tree cover than was previously the case. (What evidence would you look for to support such a statement about the palaeoenvironment?) I'm sure you're aware of other possibilities.
2) the 'megadonts': the robust, large-toothed australopiths that include Australopithecus robustus and Au. aethiopicus. Megadonty may be an adaptation reflecting a diet that's heavy on tough, fibrous plant material, perhaps foraged for in a open, dry environment.
3) our own genus, particularly erectus and its descendants, a radiation that's chararcterised by quite marked anatomical changes compared to the earlier hominins. A change in diet may be involved – there's evidence that meat became a more substantial part of the diet with erectus, and this would have an impact on the size of the digestive system, the size of the brain, and social behaviours. (There's another paper about the importance of diet, that I must write about for you.) What's more, at the time the global climate was going through alternating glacial/interglacial periods. The associated habitat changes may have both acted as selective pressures and facilitated (or hindered) population dispersal. (Think about the opportunities offered for dispersal by a drop in sea level during a glaciation, exposing parts of the continental shelf.)
As Foley says, "… the environmental context [may] involve an element of change, such as the expansion or contraction of particular habitats. … [A]daptive novelty … allows for the more extensive or successful exploitation of available habitats. This will certainly involve behavioural innovation, and most probably physiological and anatomical modifications."
D. Foley (2002) Adaptive radiations and dispersals in hominin evolutionary ecology. Evolutionary Anthropology Supplement 1: 32-37