How did we get here?
That’s not the first line in a hangover joke. It’s a question that has been asked for centuries about the origins of life on Earth.
At Harvard last week, an A-list of astronomers, physicists, Earth scientists, and chemists met in the Radcliffe Gymnasium to look at this and other fundamental questions. (What is life? Are we alone in the universe?)
The daylong conference on March 7, “Origins of Life: The Earth, the Solar System, and Beyond,” was co-sponsored by the Radcliffe Institute for Advanced Study and by Harvard’s Origins of Life Initiative.
The idea was to “ask the questions, and get the wise guidance of our distinguished guests,” said Origins of Life director Dimitar D. Sasselov, a Harvard professor of astronomy.
His own question: What is the diversity of planetary environments? (As of this year, scientists have identified 250 known planets where the conditions of life are possible.)
Sasselov also asked: Does a diversity of planets imply a “diversity of possible biochemistries?” (So far, scientists speculate that life as we know it depends on carbon, oxygen, and water.)
The Origins team, in its second year of interdisciplinary research, is a bridge between the physical and life sciences. It explores the stuff of life through a range of disciplines, from astronomy, physics, and chemistry, to Earth science, engineering, and microbiology.
New technologies in physics and biochemistry make it possible to investigate fundamental questions faster and deeper than ever, said Sasselov. The scale of inquiry can be cosmic (discovering extra-solar planets) or it can be nanoscopic (investigating the origins of ribosomes, the ancient cellular structures that make proteins).
Lucy M. Ziurys prefers the cosmic scale. She’s a professor of chemistry and astronomy at the University of Arizona, Tucson, and uses radio frequency telescopes to parse the chemistry of interstellar space.
Life on Earth began with “organic starting material” that came from outside the solar system, she said. The atmosphere of early Earth was rich in methane (CH4) and carbon dioxide (CO2) — both carbon-bearing species.
Ziurys and others like her peer into the galaxy and beyond to identify the composition of molecular gases that could have bestowed the gift of carbon to Earth. The current list of 140 interstellar molecules, she said, includes a large fraction that have carbon. Most were identified through radio astronomy, which allows experts to fingerprint the chemical spectra of giant interstellar gas clouds.
To find the origins of life, said Ziurys, “follow the carbon.” That means following the steps of stellar evolution, beginning with immense interstellar explosions called supernovae. From these evolve giant stars, including one type (Asymptotic Giant Branch stars) that ends up with carbon-rich shells.
The carbon survives the UV radiation and deep cold of planetary nebulae, the glowing shell of gases some stars form when they die. Next to evolve are molecular clouds. Dense and chemical-rich, they gave Earth’s organic chemistry a jump-start, said Ziurys.
Interstellar chemicals, processed still further in diffuse molecular clouds, make their way to planetary surfaces in the fragments of comets that rain down from space, or in meteorites. (Some contain amino acids and hydrocarbons.)
There’s interstellar dust too, said Ziurys, which to this day sifts down to Earth at the rate of 30,000 tons a year.
But life on Earth likely evolved too quickly to get the best chemical recipe for life, she said. Modern biochemistry only reflects “what was around [in space] by chance at one point” in time — a fate scientists call “chemical contingency.”
Still, life on Earth is powerful enough and pervasive enough that if intelligent beings looked from 4 billion miles away, they would conclude the planet has life here, said Massachusetts Institute of Technology planetary scientist Sara Seager Ph.D. ’99.
Her lay-friendly lecture on the search for habitable worlds wrapped up the conference of mostly complex presentations meant for experts — on space chemistry, RNA evolution, and planetary geology and planetoid atmospheric science.
For one, said Seagar, the curious aliens investigating “the pale blue dot” of Earth would detect life’s telltale signs: water vapor, carbon dioxide, and — “the smoking gun” — oxygen. They would also detect other signs of life, including changes in brightness from oceans and clouds and the “terracentric biosignature” of vegetation.
Earth and planets from outside the solar system are hard to see from far away because of blinding glare from the stars they orbit. (Earth is 10 billion times fainter than the sun, said Seagar.) She compared seeing candidate planets from space to detecting a firefly hovering six feet from a searchlight — with a telescope 2,600 miles away.
Exoplanets (those outside the solar system) are hard to find too because direct imaging is expensive, said Seagar. (Though an alternative technology is promising, and is already being used; it looks for the light signatures of planets transiting their stars.)
Scientists have only identified about 250 exoplanets, but will find more, said Seagar — in part because there are so many places to look. In our galaxy alone, there are 100 billion stars — and beyond our galaxy there are 100 billion more galaxies.
What about life on other planets? There are constraints in the universe, since life as we know it seems to require water, oxygen, and some form of chemical energy. “Complex life will be hard to find,” said Seagar — and finding microbes more likely.
INITIATIVE MAKES PROGRESS
In the past year, Harvard’s Origins of Life Initiative has made considerable progress, according to Origins director and Professor of Astronomy Dimitar Sasselov. During a meeting March 6 for scholars affiliated with the initiative, Sasselov detailed some milestones achieved during 2007.
Among other things, he said, affiliated faculty and fellows published 31 research papers and established eight courses related to the initiative. The research team continued to expand, with the addition of Martin Nowak, professor of mathematics and biology; Howard Stone, the Vicky Joseph Professor of Engineering and Applied Mathematics; Visiting Scholar Craig Venter; and Professor of Genetics George Church.
They also added to the initiative’s administrative structure, hiring a new executive director and finance administrator, and welcomed a new postdoctoral fellow.
For the coming year, Sasselov said the initiative hopes to move into new space in the Bauer Laboratory, add more courses, and recruit two new faculty members: one junior faculty with a molecular and cellular biology expertise, and one senior faculty member with chemistry and cellular biology expertise.
— Alvin Powell