For millions of years, the blue-footed boobies danced and honked, the frigate birds preened, the giant tortoises lumbered about and the marine iguanas adapted to life at sea. In the remote Galapagos Islands near Ecuador, it's an animal paradise, a tropical Eden of creatures that doesn't exist anywhere else.
As we discovered during our journey to the Galapagos, it's not just that many of the creatures here are unique, it's that they have no fear of humans.
The animals are so unguarded because for millions of years they lived here in splendid isolation. But not anymore.
Call it the invasion of the tourists.
The world, it seems, wants to see this most exotic of places. But someone forgot to tell the animals. That became clear when we got off the plane at Baltra, the main airport in the Galapagos. A giant iguana was lounging in the sun on the runway. An airport worker scurried out and gingerly chased the lumbering creature out of the way taxiing airplanes.
Clearly, protecting the fragile ecosystem and managing a growing tourism industry is a balancing act.
"Nightline" joined a group of American tourists for a few days of their Galapagos voyage to see how man and nature are co-existing 600 miles off the coast of South America in the Pacific Ocean. It quickly became clear that the remoteness that shaped the islands and allowed the animals to evolve in their different ways is no longer enough to protect them.
While some tourists stay in hotels on the main islands, most opt for the tourist boats. Our boat carries a dozen passengers. Every boat, every tourist group, must have a licensed Ecuadorian guide.
Ours is Carmen Guzman. She has a Ph.D. in biology and environmental management.
"This is the place where we can get very close to the animals, and it's really one of the few places in the world where you can notice that the animals are not afraid of people," she told the eager group of tourists.
An hour later, we were aboard small inflatable zodiacs that allowed us to navigate shallow waters and the shoreline. We got our first glimpse of exotic creatures as we motored quietly into a lagoon called Black Turtle Cove, a mangrove swamp, that is a breeding ground for the Pacific green sea turtle.
As if on cue, the turtles appeared. There were quiet whispers of awe. The click of camera shutters punctured the silence.
At sunset, our zodiacs steered towards a massive mangrove tree dotted with thousands of herons, like ornaments on a Christmas tree. The white feathers of the herons reflected the pink-orange glow of the setting sun. It was a spectacular sight.
Day two took us to an island called Rabida, famous for its red sand and its abundant wildlife. As soon as we hopped off the boats, we were distracted by a baby sea lion.
These creatures exude a cuteness that borders on seduction. The tourists flocked around, snapping pictures and gasping in admiration. The sea lion and its mates seemed content be the subject of so much admiration. And as we were beginning to learn, those sea lions had a habit of stealing the show.
A walk down the beach revealed a rookery of pelicans. Dozens of them are perched in nests along the steep hillside. Tiny chicks screeched for feed, diving into a mother's pouch when she returned with fresh fish. The tourists were enthralled. The pelicans, just feet away from us, were indifferent.
It's absolutely amazing," said tourist Karalee Brune, a nurse from Livermore, Calif. "I love pelicans, but you cannot get anywhere near this close to a pelican rookery on the mainland."
"It's unreal," agreed Rachel McMillan, 20, the youngest of the group who was touring the Galapagos with her aunt from Memphis.
"It sort of feels like, you know, I'm at the zoo or something watching an exhibit," McMillan said. "But then, I realize that this is reality. I'm in their natural habitat, watching them live their natural lives. It's incredible."
Carmen, the guide, was always watching carefully over her flock. As we toured the island, we begin to understand why the Galapagos is called a living laboratory of evolution.
There are wonders everywhere: a lava lizard here, a Galapagos mockingbird there -- and 13 different species of Darwin finches, so named because it was the different shapes of finch beaks found on each island that inspired Darwin's theory of evolution.
"We are a group of islands that are not too far from each other, but they are far from the mainland so the animals had a time to change, they adapted to a new environment, they evolved into a different way," Guzman explained. "So we have all these species of finches that look very similar to each other. However, the shape and the size of the beak changes a lot according to the kind of food that they find in the place where they live."
Roger Bower, a retired high school biology teacher from suburban Detroit, called the islands "unbelievable."
"This is the world that you talk about every day. The animals are fantastic," he said.
He teaches evolution in his class, a subject that he says is difficult to explore with some students.
"Lots of kids have the attitude that it's going to interfere with their religion or their ideals," he said.
What he is seeing on the Galapagos confirms that his teaching about evolution was right on target, Bower said.
"Things do change," he said. "Animals do change, plants change and here they change pretty rapidly."
It is easy to see why the Galapagos Islands were named the very first World Heritage Site when the United Nations began the program in 1978.
Everywhere we go, there are other boats and other tourists.
Our group leader, Barry Boyce, first came to the islands 21 years ago. Back then, there were 40,000 tourists visiting the Galapagos a year. Last year, there were 161,000 tourists.
"It's very, very typical," said Boyce. "It makes it more difficult and more challenging to organize your walk. You have to figure out where the other guides are gonna go and try to go the other way. Once upon a time, you had the islands to yourself, so it's certainly more challenging these days."
Nowhere is that more apparent than in the main city in the Galapagos: Puerto Ayora. Forty years ago there were just 3,000 residents on this vast chain of remote islands. Today, there are 30,000, and half of them are in Puerto Ayora. Yet there is no sewage system, and the trash dump gets bigger and bigger.
Officially, Ecuador doesn't allow citizens from the mainland to resettle in the Galapagos without a permit. But good jobs are plentiful on the islands and a rarity on the mainland of this very poor country, so people find a way.
The explosion of tourists and residents here is the reason why in 2007 the United Nations added the Galapagos Islands to its list of world heritage sites in danger. Alien plants, animals and insects introduced by humans threaten the fragile ecology. And humans are clearly the most invasive species of all.
"Now, it has grown so much," lifelong Galapagos resident Marco Galarz told us in Spanish. "Before, there were just a few people living here, but people keep coming and the islands are getting overpopulated."
Felipe Cruz, 50, technical director of the Darwin Research Station, was also born on the Galapagos, long before tourism became a booming industry.
"The danger list is supposed to be a tool that the World Heritage Center has set up in order to help the sites with publicity, with access to funds, with more action from the governments of the place -- but never to be a punishment at all," Cruz said. "I wouldn't put as a fair that Ecuador has messed up the Galapagos. I think that the world has messed up with the Galapagos."
Cruz insisted it is not too late to save the Galapagos.
And to be fair, Ecuador is doing a lot to protect the Galapagos.
Access is tightly controlled. The best of the tour companies are conscientious and careful. For all the problems in the towns of the Galapagos, the park itself, which is 97 percent of the land mass, is meticulously maintained. Only four of the 13 main islands are inhabited. The rest are open to a limited number of visitors during the day.
Even for tourists, this is an expensive place to get to and an expensive place to get around. Most who come here bring with them a respect for the extraordinary natural history they are witnessing up close. They are constantly reminded: Stay on the paths, don't touch or feed the animals, no flash cameras.
Susan Roux, a physician from Monterey, Calif., brought her son and grandson here. It was an unforgettable 16th birthday present for Mitchell. Susan wanted to see the Galapagos, and she wanted her grandson to see it before it disappears.
"It's tremendously fragile, and I worry very much about all these people all over it, walking so close to the animals," she said. "They seem not to mind, but there's gotta be a tipping point when it's too much.
"The more we tromp around in the sand and get really close shots of these beautiful little seal faces, I start to feel guilty," she added. "I don't want to be a bad guy and I don't want to be impacting anything that's this special, and I try really hard to do everything they tell us to do -- not get too close. But in the back of my mind, I still worry about it."
The tourists may be looking on in wonder, but the animals here seem to be just as curious about the strange creatures who wander their islands.
Make no mistake. We are the visitors here. This is their home.