In Episode Seven, The Clean Room, Neil deGrasse Tyson starts our journey traveling to the shallow seas that formed what we now know as the Grand Canyon a billion years ago in what then was the Precambrian Epoch to find the only kind of life on the planet: blue-green bacteria. Oxygen, one of the by-products of photosynthesis by microbes such as cyanobacteria and their descendants -including algae and higher plants, transformed the Precambrian Earth and made possible the evolution of more complex organisms.
Jeffrey Touchman, at the School of Life Sciences at Arziona State University, has done groundbreaking research to illuminate large gaps in the available genetic data for photosynthetic microbes through the study of organisms known as phototrophic extremophiles living in unusually harsh and exotic environments such as the buried lakes of Antarctica where a biodiversity of extremophilic anoxygenic bacteria are known to exist.
Touchman's research is focused on genome sequencing and molecular analyses of heliobacteria, proteobacteria and a cyanobacterium with the ability to shift into anoxygenic (oxygen-free) photosynthesis in the presence of sulfide, a possible evolutionary “missing link” between anoxygenic and oxygenic photosynthetic organisms.
Touchman, who is also an adjunct investigator at The Translational Genomics Research Institute (TGen), has chosen his photosynthetic, microbial partners carefully; each bears a unique metabolism, physiology or ecology and differs in fundamental ways from sequenced genomes of any other phototroph. Hidden in these organisms’ various genetic codes may be hallmarks: traces of early evolutionary innovations pointing to the origin of oxygen-evolving high-energy photosynthesis.
There are important linkages between Touchman’s work on earthbound origins and astrobiology. "Phototrophic extremophiles are excellent model microbes for studies of interplanetary photosynthetic exchange," Touchman said.
Embedded within the genetic codes of these extremophile organisms, he believes, there may be traces of early evolutionary innovations, hallmarks that led the earliest life forms on Earth to developing oxygen-evolving high-energy photosynthesis.
"Oxygen is a central biosignature or fingerprint of life sought in the atmospheric spectra of planets beyond our solar system. Detailed molecular understanding of how photosynthetic microbes can push the boundaries of extreme-environment existence on our own planet will also fill important gaps in our current understanding of extra-terrestrial potential for oxygen-evolving photosynthesis,” Touchman adds.
“Some microorganisms can survive interplanetary journeys cocooned inside rocks blasted off planets by comet and asteroid impacts. That rocky panspermia is an effective mechanism for spreading life within a planetary system,” adds the director of the ASU Beyond Center for Fundamental Concepts in Science, Paul Davies. The arrival of oxygenic photosynthesis via transport of materials by external means, such as meteorites, could profoundly change the direction of biological evolution on a planet’s surface.
Paul Davies, director of ASU’s Beyond Center for Fundamental Concepts in Science, developed some of the thinking upon which Touchman’s extraterrestrial pursuits are based. "Some micro-organisms can survive interplanetary journeys cocooned inside rocks blasted off planets by comet and asteroid impacts," he said. "That rocky panspermia is an effective mechanism for spreading life within a planetary system.
About 580 million years ago, life on Earth began a rapid period of change called the Cambrian Explosion, a period defined by the birth of new life forms over many millions of years that ultimately helped bring about the modern diversity of animals. Fossils help palaeontologists chronicle the evolution of life since then, but drawing a picture of life during the 3 billion years that preceded the Cambrian Period is challenging, because the soft-bodied Precambrian cells rarely left fossil imprints. However, those early life forms did leave behind one abundant microscopic fossil: DNA.
Because all living organisms inherit their genomes from ancestral genomes, computational biologists at MIT reasoned that they could use modern-day genomes to reconstruct the evolution of ancient microbes. They combined information from the ever-growing genome library with their own mathematical model that takes into account the ways that genes evolve: new gene families can be born and inherited; genes can be swapped or horizontally transferred between organisms; genes can be duplicated in the same genome; and genes can be lost.
The scientists traced thousands of genes from 100 modern genomes back to those genes' first appearance on Earth to create a genomic fossil telling not only when genes came into being but also which ancient microbes possessed those genes. The work suggests that the collective genome of all life underwent an expansion between 3.3 and 2.8 billion years ago, during which time 27 percent of all presently existing gene families came into being.
Eric Alm, a professor in the Department of Civil and Environmental Engineering and the Department of Biological Engineering, and Lawrence David, who recently received his Ph.D. from MIT and is a Junior Fellow in the Harvard Society of Fellows, have named this period the Archean Expansion. The image below shows the exceptional folding exposed in Archean age gneiss in Southeast Greenland.
Because so many of the new genes they identified are related to oxygen, Alm and David first thought that the emergence of oxygen might be responsible for the Archean Expansion. Oxygen did not exist in the Earth's atmosphere until about 2.5 billion years ago when it began to accumulate, likely killing off vast numbers of anerobic life forms in the Great Oxidation Event.
"The Great Oxidation Event was probably the most catastrophic event in the history of cellular life, but we don't have any biological record of it," says Alm.
Closer inspection, however, showed that oxygen-utilizing genes didn't appear until the tail end of the Archean Expansion 2.8 billion years ago, which is more consistent with the date geochemists assign to the Great Oxidation Event.
Instead, Alm and David believe they've detected the birth of modern electron transport, the biochemical process responsible for shuttling electrons within cellular membranes. Electron transport is used to breathe oxygen and by plants and some microbes during photosynthesis when they harvest energy directly from the sun. A form of photosynthesis called oxygenic photosynthesis is believed to be responsible for generating the oxygen associated with the Great Oxidation Event, and is responsible for the oxygen we breathe today.
The evolution of electron transport during the Archean Expansion would have enabled several key stages in the history of life, including photosynthesis and respiration, both of which could lead to much larger amounts of energy being harvested and stored in the biosphere.
"Our results can't say if the development of electron transport directly caused the Archean Expansion," says David. "Nonetheless, we can speculate that having access to a much larger energy budget enabled the biosphere to host larger and more complex microbial ecosystems."
David and Alm also went on to investigate how microbial genomes evolved after the Archean Expansion by looking at the metals and molecules associated with the genes and how those changed in abundance over time. They found an increasing percentage of genes using oxygen, and enzymes associated with copper and molybdenum, which is consistent with the geological record of evolution.
"What is really remarkable about these findings is that they prove that the histories of very ancient events are recorded in the shared DNA of living organisms," says Alm. "And now that we are beginning to understand how to decode that history, I have hope that we can reconstruct some of the earliest events in the evolution of life in great detail."
For more information: “Rapid evolutionary innovation during an Archean Genetic Expansion,” by Lawrence A. David and Eric J. Alm. Nature online Dec. 19, 2010.
Discovering the the Age of the Earth
Amidst sub-zero temperatures, Geochemist Clair Patterson's men dug deep into the Antarctic ice to unearth snow that fell before the start of the Industrial Revolution. Patterson developed the uranium-lead dating method into lead-lead dating, and by using lead isotopic data from the Canyon Diablo meteorite, he calculated an age for the Earth of 4.55 billion years; a figure far more accurate than those that existed at the time and one that has remained unchanged for over 50 years.
Patterson worked under the assumption that meteorites are left-over materials from the creation of the Solar System, and thus by measuring the age of one of these rocks the age of the Earth would be revealed. Gathering the materials required time, and in 1953, Clair Cameron Patterson had his final specimens from the Canyon Diablo meteorite. He took them to the Argonne National Laboratory, where he was granted time on a late model mass spectrometer.
In a meeting in Wisconsin soon afterward, Patterson revealed that the definitive age of the Earth is 4.550 billion years (give or take 20 million years).
Patterson had first encountered lead contamination in the late 1940s as a graduate student at the University of Chicago. His work on this led to a total re-evaluation of the growth in lead concentrations in the atmosphere and the human body from industrial causes and his subsequent campaigning was seminal in the banning of lead additives to gasoline and lead solder in food cans.
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