an opposable thumb is a feature of primates which allows them to climb tree branches better, but hominids took this much further to evolve a fully opposable thumb with far more functionality
but it is really the brain combined with bipedal gait to make the most use of this thumb that sets humans most apart from other species and the modern brain requires 20% of our blood supply and energy
as the brain evolved larger and larger (modern brain is 3x the size of chimpanzees which is already larger than most other species in relation to body weight), these energy requirements were met any several ways and with a number of ramifications
primates had evolved as hind gut fermenters and had diets consisting mainly of “C3” vegetation such as foliage from which they extracted the needed protein and energy by having to grind away eating for 50% of their waking hours and thus they had large molars and massive chewing muscles (masseters).
primates supplement with fruits in season but these whilst providing energy do not provide protein or fats and sometimes supplemented with raw meats (small primates ate insects for protein but this is inadequate for large primates)
this raw vegetation and even raw meat was not sufficient to provide the energy and oxygen demands of the enlarging brain
even now for adult women, if their diet was solely raw foods even if they included raw meat, the poor energy bioavailability of uncooked foods results in low BMI and 50% will have amenorrhoea - clearly this diet is NOT conducive to evolution if the species can't procreate efficiently
at some point they would have discovered distal limb bone marrow and the brain were relatively safe to eat as not only did these have a higher fat content, but were less accessible to bacteria after death
a key enabler for the early hominids was getting control of fire to cook, create smoke to given them access to honey, and to keep them warm at night. This was supplemented by the use of stone tools from as early as 3.3 million years ago thanks to early hands capable of limited throwing and clubbing, which perhaps drove evolution of the fully opposable thumb (perhaps 1mya with H. habilis and H.erectus) which then allowed powerful capability to throw objects and use clubs which were useful for hunting - by 500,000yrs ago, they had developed spears
cooking foods increased bioavailability of energy by about 50% and markedly reduced the need for fermentative hind gut as less foods were passed undigested, this allowed the hind gut to be reduced in size and divert its blood flow to the brain, further allowing the brain to evolve in size.
cooked foods meant that they needed to spend FAR less time chewing and eating and thus had far greater time to develop brain creativity and consider other activities
critically, the ability to cook meant that meat could be rendered far safer to eat if it was starting to decay, less chance of acquiring an helminth infection, and no longer did they need to rely upon bone marrow alone or early kills
red meat also was a far more effective source of iron which was increasingly critical for delivering oxygen to the brain as well as providing important nutrients such as critical unsaturated fatty acids which hominids can't synthesize for brain development and vitamins such as B12 which is essential for nerve development and functioning.
the growing increased emphasis on meat drove them to become game hunters and various features needed to occur to enable this:
bipedal gait allowing hands free to attack game
ability to run and tolerate the hot African conditions compounded by an increasing hot brain needing cooling (the larger brain essentially became a 20W heater enclosed in bone) required losing their fur (the fur was no longer needed for warmth now that they could make fires to keep them warm at night) and evolving naked skin which could perspire, but this unprotected skin needed to also evolve deep melanin pigmentation to protect from UV radiation and increasing blood circulation to dissipate the heat from the brain as well as provide glucose and oxygen in high quantities (hence larger carotid arteries entering the skull).