Poachers forced this rhino subspecies to two. Scientists are in a race to save them.

NANYUKI, Kenya -- Najin and Fatu, a mother and daughter rhinoceros pair, spend their days grazing on short grass in their enclosure at Ol Pejeta Conservancy. They're surrounded by familiar humans they've come to trust; some are caretakers dedicated to keeping them healthy. Others are armed guards ensuring their protection from poachers and any other potential threats.

Najin and Fatu are the last two northern white rhinos left on earth.

The two rhinos don't pay much attention to the rotation of journalists and photographers who also come to visit, taking pictures as the world waits to learn the future of this critically endangered subspecies. Najin and Fatu simply go on with their routine of grazing and frequent daytime naps.

Thousands of miles beyond their Kenyan enclosure is a team of worldwide scientists who are determined to save the subspecies one innovative procedure at a time.

Dr. Thomas Hildebrandt is one of the scientists on a mission to save the northern white rhino. “My internal goal and hope is that we will be successful, in three years we will see the birth of a northern white rhino,” he said in a phone interview with USA TODAY from Berlin. “The overarching idea is that we have in 20 years a solid population of northern white rhinos which can be reintroduced in the wild.”

Some of his colleagues think his timeline is ambitious. Rebounding a population back from just two animals has many uncertainties.

The most endangered mammals on Earth

Northern white rhinos, a subspecies of white rhino, used to roam throughout countries in the central region of Africa, including Democratic Republic of the Congo, Uganda, Chad, Sudan and Central African Republic. Armed conflict in the region left the rhinos vulnerable for decades. Poachers took advantage of illegal demand in parts of Asia for rhino horn.

"In those countries, there was civil war. So most of this rhino was affected by the civil war in those countries, [and] poachers taking advantage of killing nearly most of them because there wasn't good security for them," explains Zacharia Mutai, head caretaker for Najin and Fatu at Ol Pejeta Conservancy.

In the 1960's, there were 2,000 living in the wild according to Ol Pejeta Conservancy. By the 1980's, the population had been obliterated to only 31 living in Garamba National Park in the Democratic Republic of the Congo.

"And the number declined until there's only two left now," Mutai said.

Mutai has cared for Najin and Fatu as long as they've resided at Ol Pejeta. The two female rhinos arrived in 2009 along with two male northern white rhinos, Sudan and Suni. They were relocated to Kenya after living in a zoo in the Czech Republic for years in hopes that the rhinos would procreate at a faster rate if they lived in a climate that was more like their natural habitat. Even though the rhinos mated, they never produced a new calf.

Suni died in 2014. Four years later, Sudan, the last male northern white rhino on earth, died from natural causes at the age of 45.

But those most passionate about saving the subspecies did not give up hope.

Innovating IVF techniques for rhinos

In vitro fertilization procedures are the same in principal whether it's performed in humans or other animals -- mature eggs are extracted from a uterus, fertilized with semen in a lab and then implanted back into a uterus in hopes that one or more of the fertilized eggs will implant and create a viable pregnancy. But reproduction in rhinos is complicated; adapting the in vitro fertilization process to rhinos has its own unique challenges.

"We don't know what all the pitfalls are going to be," said Dr. Barbara Durrant, director of reproductive sciences at the San Diego Zoo. She's working on IVF techniques for white rhinos along side Hildebrandt in Berlin. "We don't know how difficult it will be. Just growing embryos in culture is very difficult."

Durrant says her team prefers a slow and steady approach to the techniques they're creating.

The scientists from the San Diego Zoo in California are part of an international team from Germany, South Africa, Japan and other countries who are trying to innovate IVF technology that could save the northern white rhino.

There's also a team at the San Diego Zoo pursuing stem cell research to help create sperm and eggs from a more genetically diverse set of northern white rhino cell lines.

For the scientists pursuing IVF techniques, they're working with a collection of semen from four northern white rhinos: Sudan, Suni and two additional male northern white rhinos that once lived at the San Diego Zoo.

They're testing fertilization and implantation techniques in southern white rhinos, the northern white rhino's closest relative, to make sure they have the process right before trying to create the world's first northern white rhino IVF baby.

Visiting Najin and Fatu

In 2014, Najin, 29, and Fatu,19, were both diagnosed as incapable of naturally carrying a baby. Najin has a deformity on her hind legs that makes her unable to carry a pregnancy to full term. Doctors discovered Fatu had major changes in her endometrium, the inner layer of her uterus.

Both rhinos had functioning ovaries that Hildebrandt and his team hope are still viable. They are getting the necessary permits to go to Ol Pejeta to collect eggs from the rhinos.

If all goes well, they hope to transfer fertilized eggs into southern white rhinos that could carry a baby to term. They haven't figured out which cows would make suitable surrogate mothers, but the scientists made a promise that the first southern white rhino surrogate will be a cow in Kenya.

Hildebrandt also believes it will be valuable for a surrogate mother to live in close proximity to Najin and Fatu. If a calf is born, the IVF baby would be able to glean social knowledge from Najin and Fatu to help the subspecies survive.

“We think that’s very a crucial element," Hildberandt said. "We can preserve the genome by this technology we have in mind. But the learning process, that can only be practiced by living northern white rhinos and we only have two and both are in very advanced age.”

A bail-out from science

The scientists involved in this ambitious project see how their work may one day help other endangered species in need of repopulation. But the most important work is in ensuring that animals never get this close to extinction.

"When humans have been foolhardy enough to allow the population to get to this size, this is the only answer, but we are working diligently on a number of fronts to make sure that as few species as possible ever get to this stage," Durrant said. "Certainly we don't want to be so distracted by this project, the heavy science aspect of this project that we forget about natural breeding and conservation in the wild."

“The rhino didn’t get extinct because it failed at evolution," Hildebrandt said. "It got extinct because it’s not bullet proof.”

And now developing new science techniques are the only way to try to bring it back from the brink of extinction.

This story was made possible through the International Center for Journalists' Bringing Home the World fellowship.

This article originally appeared on USA TODAY: Poachers forced the northern white rhino down to two. Scientists are in a race to save them