It is the most remarkable and important time in the history of life on Earth -- 540 million years ago, when 3 billion years of simple, single-celled life reached a dramatic turning point, and life evolved into a wide variety of multi-cellular forms. It was a planetary biological event that is known as the Cambrian Transition and is sometimes even called "the Cambrian Explosion."
"All hell broke loose in evolution," said Arizona State University biologist James Elser. "All kinds of new multi-cellular life forms arose in all the major groups."
"It was a huge, huge event in the biosphere," added ASU paleontologist and astrobiologist Jack Farmer. "We switched over from a microbially dominated world to one that included large, multi-celled animals and plants -- large macroscopic organisms. It was an evolutionary event that changed whole ecosystems. Suddenly the biosphere added herbivores and carnivores to ecosystems where none had ever existed before. This meant that all the ecosystem dynamics -- energy flow processes and so on -- underwent a radical change."
"It's probably the most important event in the history of life and we don't really understand why it happened," Elser pointed out. "Though there are lots of ideas about what might have occurred."
Farmer, a professor of geology who is an authority on ancient bacterial fossils called stromatolites, and Elser, a biology professor who studies ecosystem dynamics, are particularly interested in this ancient ecological break-though event because of another role that they both play as scientists -- their role as astrobiology researchers. Farmer is director of ASU's Astrobiology Program and Elser is a co-investigator in its research initiative. ASU itself is one of five university members of NASA's Astrobiology Institute (NAI).
Elser is also the principal investigator for a new $750,000 NASA research grant aims to define and try the still-unproven environmental, ecological and evolutionary controls behind the major change in Earth's biosphere during the Cambrian Transition. The grant, which is one of eight new supplementary research awards given out by the NAI, is entitled "Evolution in Microbe-based Ecosystems: Desert Springs as Analogues for the Early Development and Stabilization of Ecological Systems."
According to Elser, the new research is aimed at studying the ecology of a set of remarkable desert springs located near Cuatro Cienegas, Coahuila, Mexico which may give scientists a modern "analogue" for conditions during the transition from Earth's ancient bacteria-based biosphere to the present, more complex one.
The area's numerous clear-blue, spring-fed pools and outlet streams each have their own unusual thermal and chemical conditions and their own distinctive biota that have evolved to cope with the specific environments. Like a Galapagos Archipelago in reverse, these islands of water set in a sea of desert give the research team a range of extreme environments and their accompanying food webs to study in relation to the Cambrian Transition.
As the Galapagos' model gave Darwin insights into the workings of evolution, the astrobiologists hope that Cuatro Cienegas's aquatic environments will help them model the dynamics of the evolution of Earth's biosphere at the start of the Cambrian Period -- evolutionary and ecological knowledge that is, in turn, vital in the search for life on other worlds.
"Part of what astrobiology is trying to do is to calibrate what the targets for life are," explained Farmer. "One thing that we've learned is that complex, multi-cellular life forms -- which are a necessary step on the way to intelligent life -- developed very late in the Earth's 4.5 billion year history. . .
"It's a natural thing to look backwards and ask how the biosphere emerged on our own planet and what were the big events that characterized its evolutionary history -- how repeatable are those events? What are the elements that may be universal and could be applied to the search for biospheres on other planets?"
At the moment, Farmer and Elser point out, much remains uncertain about the Cambrian Transition.
Between 3.5 billion years ago and 600 million years ago, the geological record yields only bacterial evidence -- particularly in the form of stomatolites, reef-like mineral deposits left by aquatic bacterial colonies. "They're the most common fossil on Earth," said Farmer.
Then the stromatolites largely disappear and are followed by a multitude of "metazoans," or multi-cellular life forms -- at the Cambrian Transition. "People theorize that the metazoans may have grazed the stromatolites away" said Elser. "The only place where you can still find them are places where the chemistry or physical environment is too extreme for higher animals to live."
What caused or allowed more complex life to evolve and dominate is an even more difficult mystery. Farmer points to a slow build-up of oxygen in the environment to levels conducive for the oxygen-based, high energy metabolism of higher organisms. According to this scenario, photosynthetic cyanobacteria pumped oxygen into the environment for billions of years until they could overwhelm the oxygen-bonding chemistry of the early ocean and land surfaces and then build up to the threshold level of atmospheric oxygen required for complex life.
Elser, however, posits a different possibility. "One hypothesis we're testing is that the reason metazoans were unable to evolve was that they had trouble grazing stromatolites -- the food quality was lousy and not nutritious enough to support higher animals," said Elser. "Perhaps there was an environmental change that changed the nutrient composition of the stromatolites and made them more edible and nutritious for grazers, so the grazing lifestyle could evolve."
One of the issues that interests Elser in considering the dynamics of a microbe-based food web is the relative abundance of available phosphorus, a fairly rare element on Earth, but a requirement for ribonucleic acid (RNA, which is used intensively by the machinery of cellular growth and reproduction in complex organisms), as well as other key biochemical constituents in organisms.
The point that both the geologist and the biologist make is that the issue is certainly complicated, as it involves the elaborate dance of early planetary chemistry with the workings of a largely unknown prokaryotic (simple-celled) biosphere.
These issues make Cuatro Cienegas a sort of natural laboratory for testing key hypotheses about Earth's earliest days. Cuatro Cienegas' springs and streams are one of the few places remaining on Earth where stromatolite-forming bacterial mats still flourish and form the base of the ecosystem. The varying temperatures and chemistries of the different pools also provide the makings for a 40,000-year-old experiment to test the effects of these environmental variables on ecosystem evolution.
"It's an interesting set of environments with a lot of extremes and variations in the chemistry," said Farmer. "There are variations in the water chemistry, in the alkalinity and in the thermal regime from pond to pond and this creates an environmental mosaic, a checkerboard of environments. Within a very short walking distance you can have totally different conditions. Nature loves to take advantage of situations like that by populating different environments with different species. Almost every pond has its own endemic flora and fauna."
To help make sense out of the data coming from these varied environments and apply it to the geological record, Elser has assembled a multi-disciplinary team that includes himself, Farmer, microbial ecologist Ferran Garcia-Pichel, paleontologist Carol Tang, theoretical ecologist William Fagan and ichthyologist and ecological authority on Cuatro Cienegas , W.L. Minckley. Also playing a key role are Valeria Souza and Luis Eguiarte, evolutionary biologists from the Universidad National Autonomidad de Mexico .
A multi-disciplinary effort such as this is an increasingly common research scenario in contemporary ecology, Elser points out, but ecological study itself is a late entry in the field of astrobiology, which has traditionally worried more about the life requirements of the beginnings of life more than those of more complex organisms that came later. The answer to finding life on other worlds, however, may be to search for the second-stage environment.
"Most of the past work in astrobiology is focused on the physical and chemical conditions necessary for the first evolving chemical proto-biological systems to appear," noted Elser. "Our argument is that it is one thing for that to appear, and it's another thing for it to persist and to evolve and expand. That becomes an ecological question.
"It's possible for a biological system to appear, and then blow itself out. If you want to go look for life on places like Mars, you have to find a place where life has persisted and become big, or widespread -- so you can have some hope of finding some remains. This is where the biggest hope for finding life is, I think, and this is where our research is going."