Salt of the Early Earth
Scientists have long assumed that life originated in the sea. If life did spring from salt water, that could explain why all organisms use salt. But Paul Knauth, an astrobiologist with Arizona State University, says while we always assume that life came from the ocean, this theory has never been proven. He suggests we need to consider the possibility that life originated in fresh water.
"Fresh" water is somewhat of a misnomer - all fresh water bodies still do contain some salt. Non-marine salt levels are less than 1 part per thousand, while marine salt levels are around 35 parts per thousand. But when life first appeared around 3.5 billion years ago, the ocean was much saltier than it is today. Estimates of the early ocean's salinity range between 1.2 to 2 times present-day salinity.
"Life is stressed today in the current ocean, so one can speculate that higher salinities make things even tougher," says Knauth.
Salt does seem to have played some sort of role in the origin of life - it is the precise concentration of salts that is at issue. In constructing the steps that led to the first life form, many scenarios invoke the concentration of salts through evaporation.
"In the early saltier ocean, this would lead to a real devil's brew," says Knauth.
However, non-marine bodies of water have a wide range of changing environments. Knauth says that some of these fresh water environments probably had the optimal salinity for the kinds of molecular assembly proposed for the origin of life.
Shiladitya DasSarma, a professor at the University of Maryland Biotechnology Institute, Center of Marine Biotechnology, agrees that life could have originated in fresh water pools. So long as these pools had a certain amount of organic molecules, prebiotic evolution could have occurred. However, DasSarma thinks that life also could have begun in the early salty ocean. He has found that, due to the low water activity of hypersaline brines, macromolecules can form from organic molecules. A macromolecule is a very large molecule, such as a protein or other polymer.
The macromolecules in these salty waters, combined with other molecules, could have formed membranes capable of Darwinian evolution (and thus be classified as a life form).
Liquid water began accumulating on the surface of the Earth about 4 billion years ago, forming the early ocean. Most of the ocean's salts came from volcanic activity or from the cooled igneous rocks that formed the ocean floor.
This volcanic activity also created island chains that grew over time. Tectonic plate movement caused these islands to collide, forming the cores of the continents. The continents developed fresh water lakes and ponds through rainfall and other meteorological processes.
Soon after both salty water and fresh water were available, life originated. The oldest fossils we have are from 3.5 billion-year-old cyanobacteria, but life probably emerged even earlier than that. Genetic analysis has shown that the archaean branch of life came first, appearing sometime before bacteria.
The specific antiquity of halophiles is not currently known, but because they breathe oxygen they are not believed to be one of the earliest forms of archaea. Oxygen wasn't a major component of the Earth's atmosphere until anaerobic organisms like cyanobacteria began producing it. However, DasSarma has some evidence that halophiles may lie very deeply in the tree of life.
DasSarma and his team have recently sequenced the genome of an extreme halophile called Halobacterium species NRC-1. DasSarma says that when the genes of Halobacterium NRC-1 are compared to other organisms, this halophile seems to be the most ancient archaean.
"This is very unexpected," says DasSarma. "The small ribosomal RNA-based trees pointed to halophiles as recent relatives of a class of anaerobic archaea called methanogens, which have very simple metabolism involving methane production from inorganic gases."
DasSarma says the close relationship between halophiles and methanogens never made sense because they do not share physiological capabilities: Halophiles need oxygen; methanogens do not. But it turns out halophiles are able to produce energy without oxygen in two ways: from the degradation of arginine, and by using the photosynthetic molecule bacteriorhodopsin.
Perhaps these two methods of non-oxygen energy production are the last remnants from the halophile's earlier, anaerobic days. As the Earth's oxygen levels rose 2 billion years ago, the gas would have killed off many anaerobic organisms. In a process called "lateral gene transfer," halophiles may have borrowed genes from aerobic bacteria in order to survive this increase in oxygen.
"Our analysis of genes in halophiles suggest common ancestry with many bacterial genes, for example, those involved in aerobic respiration," says DasSarma. "Whether these are recently acquired by lateral gene transfers or have common ancestry with bacteria is currently being analyzed."
The rise of oxygen as an atmospheric gas changed the face of life on Earth. Many life forms died out, while other life forms adapted to the new gas. But Knauth says the early ocean wouldn't have absorbed very much of this oxygen. If the ocean was warm in its early days - and Knauth believes that the ocean 3.5 billion years ago was like hot tap water - then the combination of high temperature and high salinity would have resulted in an ocean with very little dissolved oxygen.
Oxygen-use has been linked with the development of complex life forms. Therefore, Knauth says the ancient, anoxic sea would have housed only the simpler organisms like anaerobic bacteria, while aerobic organisms and other complex life forms evolved in fresh water. But another dramatic environmental change was on the horizon: the formation of the continents led to a process that reduced the amount of salt in the ocean. Low-lying continental areas were sometimes flooded by ocean waters, but these shallow seas evaporated relatively quickly - in about 100 million years. The minerals left behind formed large salt basins, and this sequestered salt resulted in lower ocean salinity.
As the ocean cooled and salt basins began to form, the ocean would have been able to absorb more oxygen. This oxygen absorption opened up a new environmental niche for aerobic organisms, and the sea would have seen an explosion of new life forms. In fact, if the salt basins formed around 540 million years ago, Knauth believes ocean salt levels could have had a hand in the Cambrian Explosion.
Scientists still have not figured out what triggered the enormous increase in the diversity of life in the Cambrian era. But salt basins, forming in a brief period of time and decreasing the salinity of the oceans, would have had a profound impact on life.
"The currently favored view for the major control on the Cambrian explosion of life is that atmospheric oxygen levels built up until metazoan life was possible," says Knauth. "These larger organisms need higher oxygen levels to survive. My point is that it is dissolved oxygen that is critical here, not just the atmospheric level. The arrival of big salt deposits on the continents in the latest Precambrian could have been one of the key factors that allowed the shallower oceans to finally oxygenate enough for metazoans to take to the sea."
Mars originally had much more salt than the Earth, and when Mars lost 50 to 90 percent of its water through evaporation it became even saltier. The Panspermia theory says that life originated elsewhere and then was transferred to Earth by meteors. If the earliest life forms were halophiles, says Knauth, then perhaps we are really Martians.
DasSarma finds the idea of halophilic life on Mars a fascinating concept. He says it may be possible to look for such life on Mars today.
"If indeed Mars is salty and life could have evolved there, it may still be trapped in brine inclusions within salt crystals," says DasSarma. "Another property of earthly halophiles that may have some bearing on their ability to survive is that these organisms are extremely resistant to solar radiation, and therefore would be excellent candidates for interplanetary travel."
DasSarma suggests it may be possible to discover what halophiles were like in their early days by studying salt bitterns: hypersaline brines that are left after the commercial production of salt. Like the early ocean, salt bitterns are anoxic as well as extremely salty.
"It is intriguing that the intracellular salt concentrations of modern halophiles resemble the potassium-enriched, sodium-depleted bitterns remaining after the harvesting of marine salt," says DasSarma.
DasSarma says it may be possible to create a "prebiotic soup" of organic and inorganic components along with brine from a bittern. This mixture perhaps could allow growth of modern halophiles exhibiting some of their primordial capabilities.
Knauth, meanwhile, is working on the question of whether life evolved in the ocean and adapted to lower salinity environments, or whether life evolved in fresh water and then adapted to life in the oceans. He is looking at the fossil record of various non-marine environments to try to answer this question, and has found some very promising sites in Australia.
"Currently I'm exploring life on land in the Precambrian," says Knauth. "I'm looking at non-marine environments to see if the fossil record indicates whether life could have originated in that environment rather than in the sea, as we've always thought."