Out in the Mexican desert, surrounded by mountain ranges on all sides, sits a butterfly-shaped valley known as Cuatro Cienegas. Every Spring Break, about 60,000 students descend on this quiet area to enjoy the more than 200 hot springs that dot the starkly beautiful landscape.
For the rest of the year, scientists make up the majority of the tourist trade in this singular valley. For the last several years, this has included research teams working for NASA's Astrobiology Institute (NAI). One of the several goals of the NAI is to discover how life on Earth may have evolved. And this site may indeed have something to say about that.
"This is an environment dominated by microbes," says Janet Seifert, a researcher with Rice University in Texas. "Even large mammals are sort of missing here."
We tend to think of life as trees and flowers, butterflies and wolves. But the microbes - single-celled organisms too small for us to see without the naked eye - were here first, and that means bacteria and their often-extreme cousins, the archaea.
Archaea and bacteria were the only life on Earth until about half a billion years ago. The dominant fossils from that distant time are stromatolites - the fossilized remains of colonies of these ancient microbes. Coming in a variety of shapes - domes, columns, even plain, flat sheets - they are often all that remains of ancient life.
"Stromatolites are the oldest fossils we have," Jim Elser, a researcher from Arizona State University, said. "They were the dominant thing in the fossil record for a couple billion years, then they disappeared, pretty much."
Living stromatolites survive in only a very few locations today, most of them hyper-saline enclaves like Shark Bay, Australia, where metazoans - animals larger than microbes - cannot survive. It is thought that this is because animals like snails, which eat bacteria, destroy the colonies before they can form. But in Cuatro Cienegas, stromatolites and metazoans do indeed co-exist. To a scientist like Jim Elser, that makes these springs a testing ground of unrivaled potential.
"The real question is," Elser says, "what took them so long? I mean, why did it take two billion years to evolve herbivores capable of eating stromatolites?" Elsner thinks he has the answer: phosphorus, an element essential to life, was limited in the environment until then.
Elser studies something he calls "ecological stoichiometry." Stoichiometry originally comes from chemistry, and says that, for instance, every water molecule must have two hydrogens and one oxygen. Ecological stoichiometry says that each living being needs specific elements in specific quantities. Humans need oxygen to breath, calcium for bones, and so on. Without those elements available in the environment, you cannot have that particular form of life.
Scientists in the field believe that something in the environment was not available in high enough quantities to support these larger beings, prior to the so-called "Cambrian Explosion" of diverse and complex life forms. Most scientists believe this something was oxygen. But not Elser.
Elser thinks the missing powerhouse was phosphorus - an element essential to our DNA, as well as our bones - and he developed that idea based on his findings at Cuatro Cienegas.
Phosphorus levels are quite low in Cuatro Cienegas. Elser's idea was that the metazoans were not getting enough phosphorus to really do well, only enough to survive. So, even though the snails were grazing, they never got big enough or numerous enough to demolish the stromatolites.
"So the first year, we added phosphorus and determined that the snails did in fact grow better, ... but in the second year, adding more phosphorus actually made the snails grow more slowly and killed then. So, that's why we had to go down there the third year!" Elser laughs, clearly happy to have had another chance to make the trip.
Elser refers to this concept as a "stoichiometric knife's edge." In other words, for phosphorus - and other elements - there is an optimum level for metazoan survival.
Too little phosphorus and the animals don't grow. Too much and they die of an overdose. There are plenty of parallels in our own lives: Eat too little, starve to death, eat too much, die of diabetes complications. Everything in moderation, as the Greeks said. And that means phosphorus.
Elser believes that phosphorus levels were a key trigger of the Cambrian Explosion.
"The pre-Cambrian oceans were extremely low in phosphorous, and then ... there was this huge spike," Elser said.
In fact, Elser theorizes that the very first organisms that developed during the Cambrian Explosion went extinct soon afterwards because they got too much phosphorus.
"They're replaced by organisms that are capable of sequestering phosphorus in the form of apatite, which is what we make our bones from. So originally, we're arguing in the paper that apatite formation was a mechanism of detoxifying that excess phosphate."
Elser doesn't expect the theory to be terribly popular.
"It's a pretty bizarre proposal," he says with a laugh. "I think it's relatively unique!"
Be that as it may; the low-phosphorus, microbe-dominated environment at Cuatro Cienegas is not only of interest because of stromatolite grazing - or lack thereof. In fact, it's critical for an entirely different set of ideas relating to the origins of life - the construction of the RNA Tree of Life (see sidebar.).
The RNA Tree of Life is like a huge family tree, containing (ultimately) all living organisms on Earth. The idea is to figure out who is the oldest, hoping that that will tell us important facts about the last common ancestor to all life on Earth. There are three main branches - the three Domains of Life, Archaeabacteria (archaea), Eubacteria (bacteria), and Eukaryotes (everybody else). And as far as the Eukaryotes go, it's probably pretty good.
But there may be a problem with the bacteria and archaea.
The RNA Tree of Life is dependent on the idea that genetic material is passed only from parent to child, and so if two organisms share DNA, then they must be at least distant cousins. But bacteria and archaea can share DNA by horizontal gene transfer - literally, passing genes to your next-door neighbor, rather than just your child - which may muddy the waters quite a bit.
"The whole controversy in my field has been 'how much horizontal gene transfer actually goes on?'" Seifert said. "It's one thing to say they take up genes, but that places a burden on the cell, too. How much DNA can a cell take up and still be beneficial?"
If bacteria and archaea have shared significant amounts of genetic material, species that appear to be closely related may not be related at all - one species just happened, at some point in the past, to steal from the other one the particular strand of DNA that was used for the comparison.
It also isn't known whether ribosomal DNA - rRNA, the strand that is generally used for constructing the Tree of Life - specifically is actually transferred with any frequency.
"It works in the lab. You can assemble a functional ribosome from pieces. But it looks like in nature, it just doesn't happen very often," said Seifert. "And there are several reasons why it might not."
But the bottom line is, scientists are not sure. So where does Cuatro Cienegas fit into this?
That goes back to the phosphorus.
"There's a low phosphorus content in the pools," said Seifert. "That should make the bacteria in there more readily evolved to take up the DNA out of the environment, because they're going to need it."
In other words, if rRNA transfer is going on anywhere, you might expect to see it in Cuatro Cienegas.
Seifert's research is on-going; Elser's group is applying for new grants. Colleagues at the National University of Mexico continue to study and catalog the waters, and the life that lives in them.
They may be working against time.
"About two thirds of the valley is protected, but some of the pools are open to the public," said Seifert. "It's a popular Easter Break destination in Mexico."
Some of the land is owned by the Mexican government; the Nature Conservancy has also been involved in purchasing tracts of land for protection. But gypsum mining and alfalfa farming around the edges threaten even the protected regions. Wells sunk for machinery and irrigation tap into the source of water for the pools, possibly draining them or changing the flow.
"The source of these waters, the flow, is not well understood," Seifert said. "There's a lot of spatial variation; you can have a really salty pool and ten meters away have one that's almost fresh water... We don't understand how they're all connected."
Seifert hasn't seen any real changes in her study site yet, but there have been obvious drops in the water levels over the last 50 years. Her hope, echoed by Elser, is that the Mexican government will be able to protect and preserve this beautiful, unusual, site.
"Who thinks of bacteria as being beautiful, right?" Seifert asks. "But it's really spectacular what they do! The world that you so clearly see, the trees and the plants and stuff - there's this much bigger world at work and the planet couldn't survive without it!"
