In the middle of one of the least explored places on the planet, you can catch a glimpse of man's closest cousin.
Bonobos are thought of as possibly the only animals who resolve their conflicts not through violence, but through sex. In fact, they are also believed to be among the only animals, other than humans, who have sex for fun, as opposed to just for breeding.
Often called the "erotic apes," bonobos are 98.5 percent genetically similar to humans.
"Nightline" took the incredibly difficult journey into the heart of the Congo to meet up with Bila-Isia Inogwabini, who goes by "Ino." Ino is a researcher for the World Wildlife Fund who is pushing a controversial theory -- that this is the first place humans appeared on Earth.
He is credited with having made an incredible discovery -- a population of 2,000 of the extremely rare bonobos living in a vast stretch of the African jungle. That discovery represents a huge development for a species that some fear is headed toward extinction.
"The world did not know it existed," he said, and we were the first international reporters he took to see them.
But it wasn't easy. Over three days, we fought our way through nearly 20 miles of hot, dark jungle, menaced by man-eating ants and incredibly persistent bees.
Often, we set out before dawn. But when we finally got close, it was thrilling.
First, we heard the bonobos crashing through the trees. Then we heard their voices -- it sounded like a horror movie soundtrack.
And when we finally saw them, they looked like acrobatic cavemen.
A romantic mythology has built up around bonobos, a species believed to be much more peaceful than chimpanzees, which can be ferociously violent.
"[Bonobos] tend to solve their problems using sex, rather than violence," Ino explained.
That's right, bonobos are believed to resolve issues over food and territory not through violence, but through sex.
Aside from humans, they are the only other animals that make love for fun, not just for procreation.
"And this brings them closer to us than any other mammal," Ino said.
Bonobos have group sex, gay sex, oral sex and face-to-face sex. They've even been reported to French kiss.
Many people think the reason bonobos are so peaceful -- they can often be seen quietly grooming one another -- is that the society is run by females.
Right now, however, the incredible animals are critically endangered. They have the bad luck of existing only in one country -- the impoverished, lawless and war-ravaged Congo, where poachers are hunting them for food.
They are considered to be a delicacy "in some provinces of Congo," said Ino, "and some tribes."
We visited the Friends of Bonobos sanctuary in the Congo -- called Lola Ya Bonobo -- where nearly every animal has had its parents killed by poachers.
They do seem incredibly gentle, and human. We were even allowed to interact with the babies. While the young bonobos are playful, they are considered psychologically vulnerable without parents, and are given human surrogate mothers.
The surrogates "form a relationship with the animals, because the baby cannot live without their mothers," said one of the handlers.
Back in the jungle, Ino said he's witnessed bonobos engaged in very human-like behavior, and has even seen them cry. But perhaps most human of all is the fact that bonobos occasionally walk upright, which has led Ino to put forth his radical theory.