I'm traveling this weekend but want to notice a paper in Evolutionary Anthropology on "Human Adaptation to the Control of Fire" by Richard Wrangham and Rachel Carmody. It appears to summarize a book Wrangham published last year, Catching Fire: How Cooking Made Us Human. The subject is a bit off topic for this blog, which is why I did not review the book, but it matches several themes that recur on this blog.
The best evidence is physical. It happens that both speech and fire can point to about the same time for the oldest such evidence. For speech it is a fossilized hyoid bone from 800,000 years ago. It shows a human shape, indicating that the lineage had by that time lost the air sacs found on ape chests and limit the vocal range available to speakers. For fire, the best evidence is from 790,000 years ago at a site in Israel. But is that the end of the story?
Wrangham believes that controlled fire dates from about 1.9 million years ago with the first appearance of Homo erectus. That is also around the time I put the first word. His evidence is circumstantial: (1) "humans differ from all other species in being biologically committed to a diet of cooked food" [p. 189]; (2) we got this way because of our weak jaws which do best with soft foods and our smaller gut which reduces the effort we put into digestion and both of these traits seem to extend to early Homo erectus; (3) nighttime survival on the ground is very problematic without a fire and Homo erectus appears to the be the first in the human lineage that was anatomically forced to be terrestrial at all times; (4) cooked food gives more calories than the same amount of raw food and Homo erectus appears to have increased its available energy, supporting a more active life and a bigger brain.
Circumstantial evidence can be compelling, but stumbles if alternative explanations are possible. For example, warthogs survive nighttime predators by sleeping underground. Might Homo erectus have done the same thing? Also, would fire really keep predators at bay? It might be a good trick at first, but wouldn't stout hearted lions evolve in time with the courage to come near the fire? After all, predators patrol firelines on burning grass today, seizing prey as it tries to escape the flames. However, even if we can imagine different possible explanations for each point, the beauty of the fire paper's case is the way it takes a variety of seemingly unrelated facts and connects them with one strong idea.
The counter-evidence is that there is no fire and there should be. Fire is not like language—something that evaporates on the instant and leaves no trace of itself. Fire does leave traces, although of course, most of them do not last a million years. I find particularly dismaying the lack of evidence in a Dead Sea settlement [sic!] from 800,000 years ago. This find is unusually rich. Vegetable remains were found in one area, flint worked somewhere else, and shellfish cracked open in a third spot. This is a very detailed account, but the archaeologists do not report the presence of fire and this is "only" 10,000 years before fire is known to have existed in the same general region.
Yet the anatomical evidence for easier digestion must mean something. A great deal of paleontological work about diet depends on anatomical evidence, so the authors are not out of line when they propose cooked diets without having the physical evidence of fire to back up their claim. They make a good case, but doubters are not necessarily idiots.
Language-origins evidence is also largely circumstantial, although there is some brain and genetic evidence that fire cannot draw on.
The list of traits that are universal in humans and unknown in other living species is short, so it is always nice to have something to add. The setting of controlled fires, of course, is well known as a species-specific behavior, but the authors take that knowledge a good stride farther by writing about the importance of cooked food to the human race. Cooking is universal among humans; more, it is "obligate." We must eat cooked food or die. That seems a startling statement, but the authors state that "no cases are known to us of humans living on raw wild food more than a few weeks. Raw domesticated food can provide a sustaining diet for contemporary urban raw-foodists, but the few studies of health status all indicate that urban raw-foodists are at risk of chronic energy shortage" [p. 188] And this: "Human populations are not adapted to survive on a diet of raw wild food." 
Frankly, before Wrangham I had never seen the matter put this bluntly before. I had always thought of fire as a bit of a luxury, a cultural add-on. Presumably it began that way, as a technique to improve on food or survive a bit more comfortably or to keep away predators during the night, but we seem to have become dependent on cooking for our diet.
One difference between fire and language is that we do not have much trouble understanding why fire is unique to humans. We are the only ones smart enough to figure it out, but how smart do you have to be to come up with fire? Is it harder than Oldowan or Acheulean stone technologies? Experiments have shown that Oldowan tools can be made using just the part of the brain that was available back in Homo habilis times. I would like to see a similar investigation into what brain areas are necessary to building a fire.
Everybody seems to agree that language made humans who we are, even if people don't agree on who we are. More unusual is the idea that fire made us who we are. Why? Because it freed up chewing time, leaving us free to do other things. I love that ingenious explanation. "We spend much less time eating than nonhuman apes do. Great apes spend 4-7 hours a day chewing… humans spend less than one hour a day" 
That one really hit home with me. I once went on a super-safari through eastern, central, and southern Africa. Almost at the end, I came across a kudu munching away and I thought to myself how I had spent the past 3 months looking at wild animals all over Africa and what they all do all the time is chew. Animal life means eating. The efficient eaters like lions that have plenty of time for other things just sleep. It brought home to me how odd humans are to be devoting so much time to gossip, thinking, and doing cultural tasks with no practical benefit.
One important thing about fire not much discussed, however, is that it does add a burden: the need to find fuel. That's no trivial matter. Finding things to burn takes up a lot of time, especially if you live on the open savanna. Dry dung is plentiful, but it burns very quickly.
All in all, I'd say the case here is promising, but if you are not yet inclined to believe it, the authors will need a bit more evidence before you doubters are dismissed as just plain stubborn.