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Origin & Evolution of Life: January 2012


Fossil Record, Meet Molecular Clock

Arthropod expansion in morphological disparity following the Cambrian Explosion of Bilateria, as demonstrated by the Burgess Shale trilobite Olenoides and stem-Chelicerate Sidneyia. Image Credit: Smithsonian Institution, courtesy of Douglas Erwin.

A team of researchers including members of NAI's MIT team have married fossil records with molecular clock studies to reveal a new interpretation of the Cambrian explosion. Collectively these data allow an understanding of the environmental potential, genetic and developmental possibility, and ecological opportunity that existed before and during the Cambrian. The study compares the times of origin of major animal groups (from the molecular clock) with the times of their first appearance in the fossil record. The team shows that the major animal groups first diverged during the Cryogenian, roughly 300 million years prior to their appearance in the fossil record, and acquired the key components of their developmental toolkits early in their history. After a long lag, the groups' major ecological successes are reflected in the records of the Ediacaran and Cambrian. Their paper appears in the current issue of Science.

Earth's Early Atmosphere: An Update

Scientists from NAI's New York Center for Astrobiology at Rensselaer Polytechnic Institute have used the oldest minerals on Earth to reconstruct the atmospheric conditions present on Earth very soon after its birth. The findings, which appear in the December 1, 2011 issue of Nature, are the first direct evidence of what the ancient atmosphere of the planet was like soon after its formation and directly challenge years of research on the type of atmosphere out of which life arose on the planet.

The scientists show that the atmosphere of Earth just 500 million years after its creation was not a methane-filled wasteland as previously proposed, but instead was much closer to the conditions of our current atmosphere. The findings, in a paper titled "The oxidation state of Hadean magmas and implications for early Earth's atmosphere," have implications for our understanding of how and when life began on this planet and could begin elsewhere in the universe.

For more information: http://astrobiology.nasa.gov/articles/earth-s-early-atmosphere-an-update/

The Conference on Life Detection in Extraterrestrial Samples will be held February 13-15, 2012, at the Scripps Institution of Oceanography, San Diego, California.

Purpose and Scope

The return of samples from Mars was the highest-priority flagship in the U.S. Planetary Decadal Survey. It is also a key element in the European Space Agency's (ESA) Mars Robotic Exploration Preparatory Program to prepare Europe's contribution to the international exploration of Mars. Part of planning for a Mars sample return mission includes planning for what will happen to the samples after they have returned to Earth. One of the major scientific questions that will be asked in the analysis of returned martian samples is whether they contain indications of past or present martian life. In addition, international guidelines and agency policies dictate that Mars samples must be subjected to a program of life detection and biohazard analysis before they can be released from strict containment, to protect the environment of the Earth. A better understanding of current and possible investigation strategies and capabilities, including controls and measurements related to life detection in returned martian samples, is important to address both these concerns.

An understanding of planned or possible life detection strategies and measurements has major implications for several decisions related to requirements for the 2018 sampling rover, including strategies and requirements for avoiding contamination of the samples, and sample size needed to carry out the returned sample measurements.

Life detection strategies and capabilities are relevant to a range of scientific activities beyond Mars sample return, including origin of life investigations of both terrestrial and planetary materials. The search for fossils and remnants of early life on Earth benefits greatly from a variety of analytical techniques, and can inform efforts to detect life in planetary materials. Strategies and technologies for life detection can effectively be applied to meteorite studies, addressing questions regarding the organic constituents present in the early solar system as well as possibly shedding light on reports of possible life in meteorites that remain highly controversial.

For more information: http://www.lpi.usra.edu/meetings/lifedetection2012/

Anyone who has taken high school biology has likely come into contact with a ciliate. The much-studied paramecium is one of 7,000 species of ciliates, a vast group of microorganisms that share a common morphology: single-celled blobs covered in tiny hairs, or cilia. These cilia -- Greek for "eyelash" -- are used to propel a microbe through water and catch prey.

Today these hairy microbes are ubiquitous in marine environments. However, it's unclear how long ciliates have inhabited Earth: After they die, members of most species simply disintegrate in their watery environs, leaving behind no fossilized remains.

Now, geologists at NAI's MIT Team and Harvard University have unearthed rare, flask-shaped microfossils dating back 635 to 715 million years, representing the oldest known ciliates in the fossil record. The remains are more than 100 million years older than any previously identified ciliate fossils, and the researchers say the discovery suggests early life on Earth may have been more complex than previously thought. What's more, they say such prehistoric microbes may have helped trigger multicellular life, and the evolution of the first animals.

"These massive changes in biology and chemistry during this time led to the evolution of animals," says Tanja Bosak, the Cecil and Ida Green Career Development Assistant Professor in MIT's Department of Earth, Atmospheric and Planetary Sciences. "We don't know how fast these changes occurred, and now we are finding evidence of an increase in complexity."

Bosak and her colleagues have published the study in the October 21, 2011 issue of the journal Geology.

For more information: http://astrobiology.nasa.gov/articles/early-life-more-complex-than-previously-thought/

Oxygen's Stops and Starts

Based on studies of rock cores, a team of geoscientists that include members of NAI's Penn State Team have determined that oxygen did not appear in Earth's atmosphere in a single event. Instead, atmospheric oxygen came about in a long series of starts and stops.

The research was conducted using samples collected in the summer of 2007 during the Fennoscandia Arctic Russia - Drilling Early Earth Project (FAR DEEP). Scientists drilled a series of shallow, two-inch diameter cores and overlapped them to create a record of the Proterozoic Eon--2,500 million to 542 million years ago.

"We've always thought that oxygen came into the atmosphere really quickly during an event," said Lee Kump, a geoscientist at Penn State University. "We are no longer looking for an event. Now we're looking for when and why oxygen became a stable part of the Earth's atmosphere."

The research was published in the December 1, 2011 issue of Science Express under lead author Lee Kump.

Study challenges existence of arsenic-based life, Nature

"A group of scientists, led by microbiologist Rosie Redfield at the University of British Columbia in Vancouver, Canada, have posted data on Redfield's blog that, she says, present a "clear refutation" of key findings from the paper. But after Redfield and others raised numerous concerns, many of which were published as technical comments in Science, Redfield put the results to the test, documenting her progress on her blog to advance the cause of open science ... Redfield and her collaborators hope to submit their work to Science by the end of the month. She says that if Science refuses to publish the work because it has been discussed on blogs, it will become an important test case for open science."

- Arsenic, Astrobiology, NASA, and the Media, earlier post
- NASA Researchers Start To Backtrack on Earlier Claims, earlier post
- Snarky NASA SMD Response to Snarky Public Astrobiology Discussion, earlier post
- Weird Arsenic-Eating Microbes Discovered? Yes. Finding E.T.? No, earlier post
- Arsenic-Based Life Found on Earth, earlier post
- NASA's Astrobiology News: Arsenic Biochemistry Anyone? (Update), earlier post

Volcanic-hydrothermal flow channels offer a chemically unique environment, which at first glance appears hostile to life. It is defined by cracks in the crust of the earth, through which water flows, laden with volcanic gases are contacting a diversity of minerals. And yet - it is precisely this extreme environment, where the two mechanisms could have emerged, which are at the root of all life: The multiplication of biomolecules (reproduction) and the emergence of new biomolecules on the basis of previously formed biomolecules (evolution).

At the outset of this concatenation of reactions that led eventually to the formation of cellular forms of life there are only a few amino acids, which are formed from volcanic gases by mineral catalysis. Akin to a domino stone that triggers a whole avalanche, these first biomolecules stimulate not only their own further synthesis but also the production of wholly new biomolecules. "In this manner life begins by necessity in accordance with pre-established laws of chemistry and in a pre-determined direction", declares Guenter Waechtershaeuser, honorary professor for evolutionary biochemistry at the University of Regensburg. He developed the mechanism of a self-generating metabolism - theoretically, alas, an experimental demonstration has been lacking so far.

In the chemistry of the living world, a pair of nucleic acids--DNA and RNA--reign supreme. As carrier molecules of the genetic code, they provide all organisms with a mechanism for faithfully reproducing themselves as well as generating the myriad proteins vital to living systems.

Yet according to John Chaput, a researcher at the Center for Evolutionary Medicine and Informatics, at Arizona State University's Biodesign Institute(R), it may not always have been so.

Chaput and other researchers studying the first tentative flickering of life on earth have investigated various alternatives to familiar genetic molecules. These chemical candidates are attractive to those seeking to unlock the still-elusive secret of how the first life began, as primitive molecular forms may have more readily emerged during the planet's prebiotic era. One approach to identifying molecules that may have acted as genetic precursors to RNA and DNA is to examine other nucleic acids that differ slightly in their chemical composition, yet still possess critical properties of self-assembly and replication as well as the ability to fold into shapes useful for biological function.

According to Chaput, one interesting contender for the role of early genetic carrier is a molecule known as TNA, whose arrival on the primordial scene may have predated its more familiar kin. A nucleic acid similar in form to both DNA and RNA, TNA differs in the sugar component of its structure, using threose rather than deoxyribose (as in DNA) or ribose (as in RNA) to compose its backbone.

In an article released online today in the journal Nature Chemistry, Chaput and his group describe the Darwinian evolution of functional TNA molecules from a large pool of random sequences. This is the first case where such methods have been applied to molecules other than DNA and RNA, or very close structural analogues thereof. Chaput says "the most important finding to come from this work is that TNA can fold into complex shapes that can bind to a desired target with high affinity and specificity". This feature suggests that in the future it may be possible to evolve TNA enzymes with functions required to sustain early life forms.