Much is made of the early use of stone tools by human ancestors. Darwin saw the freeing of the hands ad co-evolving with the use of the hands to make and use tools which co-evolved with the big brain. And that would make the initial appearance of stone tools in the archaeological record a great and momentous thing. However, things did not work out that way.
It turns out that up-rightedness (bipedalism), which would free the hands, evolved in our ancestors a very long time (millions of years) prior to our first record of stone tools. The earliest upright hominids that are definitely human ancestors probably emerged either close to five million years ago or close to seven million years ago, depending on which of the current evidence you like and how you interpret it. The earliest chipped stone tools are a little over 2.5 million years ago.
Furthermore, at that time there was not necessarily any real increase in brain size. Maybe a little in one or two hominid lineages, but it is not clear which hominid lineage(s) were making stone tools in relation to the brain size and the increase in size is unimpressive to the extent that it is probably safe to say that as more fossils are found and more data analyzed it could go away.
It is true that about the same time stone tools show up (give or take a couple/few hundred thousand years) there may have been an increase in species of hominds, and/or an increase in some of the features that they shared, such as whopping big teeth and the skeletal and muscular aparatus to use those teeth. But it is also true, as Alison Brooks and I have shown in various analyses, that it is just as likely if not more likely that the appearance of stone tools in the archaeological record at that point in time is a function of how the arcaheoligcal record is formed. We beleive that it is fairly likely that chipped stone tools were already in use and simply became visible to us at this point. Maybe.
Which brings us to some very serious speculation, but what the heck: I think that what it takes, mentally or neurologically, to make this early, relatively simple stone tool technology is well within the range of capacities I can imagine for a chimp-like hominid. True, modern chimps have a hard time making stone tools, but their “hands” are not “freed” like a more bipedal hominids’ hands would be. The mental/neurological part is not so hard. In a series of experiments some years ago, started by Glynn Isaac, we had many dozen Harvard Undergraduates, who had no prior exposure to stone tool manufacture, bang rocks together (in isolation) for the sole purpose of making sharp edged pieces. All of them managed to replicate most of the products in a typical Oldowan industry in just several minutes. The collection of any dozen or so of these students’ produce includes all of the Oldowan “tool” forms.
The Oldowan is the outcome of breaking rocks.
As to the impact that Oldowan style technology would have on the life of a chimp-like human ancestor? This would probably be as important as any other single aspect of foraging strategy. I imagine they were mainly making sharp edges in order to sharpen sticks, or to cut into things (or both), which would have increased the range of possibilities for accessible foods at the same level that, for instance, cooperative hunting that we see in the Tai chimps of West Africa. Important. Not necessarily overwhelmingly important.