The tropical forest of the Congo Basin is second in size only to the Amazon, covering 700,000 square miles in six nations of equatorial Africa. Anthropologists believe the region was probably the first place on the planet where early humans began to distance themselves from other apes.

National Geographic photographer Nick Nichols calls this jungle heart of Africa The Last Place on Earth -- the title of his new book detailing his adventures in the region, including his assignment to follow the explorations of friend and fellow conservationist Mike Fay.

Fay made history five years ago when he and his crew completed a "mega-transect" across the dense heart of the Congo Basin to gather data on the health of this still-wild land, sparsely populated by humans.

Nichols first met Fay on an 1992 assignment to photograph the lowland gorillas of Africa. He caught up many times with Fay during the 18-month journey as the explorer walked from the Congo highlands to the Atlantic Ocean shore of Gabon.

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