There is a spectrum of opinions on when and to what degree our ancestors harnessed fire. The leading hypothesis is that Homo erectus was first hominid to harness fire . It is assumed that he couldn't create fire at will but only use and sustain it. But in 2009 the Harward professor Richard Wrangham shifted the time when our ancestors learned to sustain and use fire further back in time to Homo habilis the predecessor of Homo erectus. He published a book titled "Catching Fire: How Cooking Made Us Human" where he argued that fire was first harnessed by Homo habilis, who allegedly learned to cook, and as a consequence of it turned into Homo erectus . There is no solid evidence to support this claim, though. Besides, it adds nothing to clarify the evolution from apes to Homo.
On the contrary, my hypothesis that there was a species of apes that learned to sustain fire makes humans' evolution more explainable. It explains why these apes descended from trees. It is because fire cannot be kept there. It also explains the origin of bipedalism as it had an advantage in the carrying of dry wood over considerable distances. In the past the origin of bipedalism was attributed to the advantages it had in the carrying of meat over considerable distances . But for this theory to be true the apes had to be able to hunt. Otherwise what meat would they carry? The meat of dead animals or animals killed by predators (whatever left of them)? Such a meager amount of meat (if any) could hardly sustain the apes. But to hunt apes had to have weapons, coordinate their actions, etc, which came much later in evolution.
The hypothesis also answers the question which for long time intrigued researchers. Did the first bipedal apes live in trees? Some researchers argued that bipedalism did not emerge in savanna amongst apes that abandoned trees but rather was achieved in trees and then used in savanna . The others insisted that it was the other way around. My hypothesis does not support either claim. Bipedalism could not emerge amongst apes living in trees as it had no advantage there. It was definitely achieved after they descended to the ground as it had an advantage in the carrying of wood to the fires. But since these apes (even after becoming bipedal) still couldn't neither start fire at will nor preserve it against elements the fire got extinguished from time to time. In these situations the apes had no choice as to return to trees as staying on the ground without fire was mortally dangerous. They waited in the trees until new forest fires allowed them to descended again. Only after these bipedal apes (still capable of climbing trees) invented the means of reliable protection of fire against elements, could they start venturing far from forests deep into savanna. Until then they stayed close to the trees where they had to retreat when fire got extinguished by elements or died due to the lack of dry wood.
There are many other steps in humans' evolution that can be explained by the hypothesis, which so far remained unexplainable.