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Eukarya Archaea Bacteria Euryarchaeota or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of Th e Oceanography Society. Send all correspondence to: [email protected] or Th eOceanography PO Box 1931, Rockville, USA.Society, MD 20849-1931, or e to: [email protected] Oceanography correspondence all Society. Send of Th approval portionthe ofwith any articlepermitted only photocopy by is of machine, reposting, this means or collective or other redistirbution articleis has been published in Th SPECIAL ISSUE FEATURE Oceanography , Volume 20, Number 1, a quarterly journal of Th e Oceanography Society. Copyright 2007 by Th e Oceanography Society. All rights reserved. Permission is granted to copy this article for use in teaching and research. Republication, systemmatic reproduction, Threproduction, Republication, systemmatic research. for this and teaching article copy to use in reserved. e is rights granted All OceanographyPermission Society. by 2007 eCopyright Oceanography Society. journal of Th 20, Number 1, a quarterly , Volume Th e Emergence of Life on Earth BY MITCHELL SCHULTE 42 Oceanography Vol. 20, No. 1 AMONG THE MOST enduring and perplexing questions pondered by humankind are why, how, and when life began on Earth, and whether life exists elsewhere in the solar system or the universe. Attempts to answer these questions have fallen under the purview of a number of areas of inquiry, including philosophy, religion, and, more recently, dedicated scientifi c inquiry. Scien- tifi c study of life’s beginnings necessarily draws from a wide variety of disciplines. Because of the complexity of the problem, geologists, chemists, biologists, and physicists, as well as engineers and Th e Emergence of mathematicians, have all been involved in origin-of-life research. Geologic evidence indicative of biological processes dates back to around 3.8–3.5 billion years ago. Features that resemble modern microorganisms are present as microfossils in 3.5-billion-year- old rocks in Australia and are thought by some to show that life was well developed by that time. Life on Earth Carbon found in metamorphosed rocks from Akilia Island, Greenland, is isotopically light, thought by many to demonstrate that organisms were fractionating carbon isotopes 3.8 billion years ago. Much of the fossil and chemical evidence once thought with certainty to be products of biologi- cal activity has been questioned recently, however, and a defi nitive time frame for life’s beginnings remains controversial (Chyba and Hand, 2005). Regardless of exactly when life fi rst appeared, Earth was very different at the time than it is at present. The early Earth was a violent place, with rem- nants of the solar system’s formation colliding with its surface frequently until around 3.8 billion years ago. Many of the collisions would have been of suffi cient force to heat the oceans to tem- peratures above 100°C, and even to vaporize them on occasion1. In fact, the oceans are thought to have maintained temperatures as high as 70–100°C for much of Earth’s early history (Robert and Chaussidon, 2006), due in part to the atmospheric composition. The atmosphere contained no oxygen (oxygenic photosynthesis had yet to develop), and could have contained up to 10–100 bars of carbon dioxide, which would have warmed Earth’s surface and caused the oceans to be acidic (Nisbet and Sleep, 2001; Russell et al., 2005). Earth’s interior was hotter than at present due to friction from collisions, higher radioactive decay, and remnant frictional heat from its formation. Because of the high heat fl ow, volcanic and hydrothermal activity on the early Earth would have been greater than at present. It was under these geological conditions that life made its debut. 1 Th e collision of the earth with a particularly large body, roughly the size of Mars, ~ 4.4 billion years ago resulted in melting of the earth and the formation of the moon. Oceanography March 2007 43 LIFE IN EXTREME water activity. Nonetheless, organisms however, and contend that thermophily EnvIRONMENTS living at high temperatures, like those may simply represent an adaptation to The discovery in 1977 of hydrothermal found at deep-sea hydrothermal black survive the violent nature of the early systems at a seafloor spreading cen- smoker chimneys (see article by Fisher Earth (Forterre, 1998; Galtier et al., 1999; ter near the Galápagos Islands pro- et al., this issue), are some of the most Miller, 1998). vided new direction and revolutionized extraordinary examples. The current Other clues from the convergence of “origin-of-life” studies (Corliss et al., record holder for high-temperature life biology and hydrothermal geochemistry 1979). The abundance and diversity of is a microorganism known as Strain 121 support a hydrothermal origin of life. animals and microorganisms found in (tentatively named Geogemma barossii) First, there are structural and composi- the dark depths of the seafloor near the for its ability to grow at a temperature tional resemblances between iron-sulfur hydrothermal vents, far removed from of 121°C2 (Kashefi and Lovley, 2001; minerals that form routinely during hy- the direct influence of the sun’s energy Lovley et al., 2004). Strain 121 was iso- drothermal processes and the iron-sulfur (although the animals at hydrother- lated from a hydrothermal vent chimney centers of electron transfer proteins that mal vents do require oxygen, originally recovered from the Endeavour Segment are part of the core metabolism of many produced through photosynthesis and of the Juan de Fuca mid-ocean ridge, organisms (ferrodoxins) (Russell et al., extracted from seawater), naturally ~ 2200 m below sea level off the Pacific 2005). Second, the abundance of sulfur- attracted a great number of researchers northwest coast of North America. On- bearing minerals in hydrothermal sys- to study the nature of the life in such a going research in extreme environments tems could have supported sulfur-based seemingly inhospitable environment. continues to broaden our view of habit- metabolisms that are common energy While the animals include unique but ability on the early Earth and through- sources for primitive chemolithotrophic somewhat familiar forms, the microor- out the solar system. microorganisms (i.e., those that use in- ganisms inhabiting seafloor hydrother- What can these modern organisms organic chemicals as an energy source). mal systems are unusual and demon- tell us about how life began, and the The connection between early life and strate a remarkable ability not only to nature of early life on Earth? Accord- sulfur chemistry is reinforced by the tolerate such challenging environments, ing to comparisons of sequences of the reliance of biology on a variety of sulfur- but to thrive in, and in fact require, these 16S rRNA gene (Figure 1), prokaryotic bearing organic compounds, including conditions. These organisms, dubbed organisms (single-celled Bacteria and essential enzymes, amino acids, and pro- “extremophiles” for their ability to toler- Archaea that lack a nuclear membrane) teins (Schulte and Rogers, 2004). Finally, ate extremes in environmental condi- exhibiting the most primitive character- hydrothermal systems have very likely tions, exhibit a vast diversity of metabo- istics are all thermophilic (heat-loving), operated continuously on Earth since lisms and inhabit niches that were once with optimal growth temperatures above the establishment of its oceans. Their considered unsuitable for life. Our cur- 60°C. This observation suggests that the existence beneath the ocean’s surface rent understanding of extreme habitats “last common ancestor” to all life cur- would have provided safe haven for in- extends to conditions of extreme pH rently on Earth was an organism (or cipient life, protecting it from changing (< 2 and > 10), pressure (from surviv- community of organisms) that lived at solar, atmospheric, and surface condi- ing the vacuum of space to the deepest high temperature. This result, coupled to tions as Earth evolved. Thus, these sys- submarine trenches), metal content, the fact that volcanic and hydrothermal tems would have been a reliable source salinity, alkalinity, radiation, and low activity were greater on the early Earth of geochemical energy for over four bil- has led to the widely accepted hypoth- lion years (Shock et al., 1998). In addi- MITCHEll SCHULTE (schultemd@ esis that life began under hydrothermal tion, they provide a natural mechanism missouri.edu) is Assistant Professor, Depart- conditions. Other researchers disagree, for concentrating energy and chemicals, ment of Geological Sciences, University of Missouri, Columbia, MO, USA. 2 Strain 121 has been shown to tolerate temperatures as high as 130°C, but is not capable of growth at that temperature. 44 Oceanography Vol. 20, No. 1 Eukarya Microsporidia Euglena Animals Plants Fungi Ciliates Slime Diplomonads molds Green nonsulfur bacteria Bacteria Crenarchaeota Gram + Sulfolobus Proteobacteria Thermotoga Thermofilum Cyano- Thermoproteus Archaea Flavobacteria bacteria Pyro- dictium Thermococcus Methanobacterium Green sulfur bacteria Archaeoglobus Methanopyrus Halophiles Aquifex Methanococcus hydrogenobacter Last Common Methanogens Ancestor Euryarchaeota Figure 1. The universal tree of life, showing the three domains, based on comparison of the ribosomal RNA gene. The lengths of individual line seg- ments indicate the evolutionary distance in terms of the number of changes in the genetic sequence from one organism
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