I've been browsing the Smithsonian's website supporting their human origins hall. There's a nice feature about the archaeological work at Olorgesailie, Kenya, focusing on the relation between paleoenvironment and human behavior. Here's a snippet:
One intriguing indicator is a series of reddened beds found in the later part of the sequence, between nearly 800,000 and 500,000 years ago. These brightly colored patches of sediment were produced by burning of buried plant matter. In some instances, the reddened sediment is associated with melted diatomite, which required an enormous amount of heat and a complete absence of water. The reddened beds required, then, the accumulation of an abundance of swamp plants, followed by burial (only an underground fire could have produced sufficiently high temperatures to melt the silica in the diatomite), followed by intense drought. The fires may have started as lightning ignited the buried materials, much like peat fires in places today.