Phys 214. Planets and Life

Transcription

1 Phys 214. Planets and Life Dr. Cristina Buzea Department of Physics Room (Please use PHYS214 in subject) Lecture 21. Origin and evolution of life. Part I March 3rd, 2008

2 Contents Textbook pages , Conclusions on life at the extremes Origin of life Evidence towards the origin of life (Stromatolites, Microfossils, Isotopic evidence) When did early life begin and how it looked like Panspermia - the possibility of life migration from nearby planets

3 Conclusion - Life at the extremes The extreme conditions that limit the growth or prove lethal to most organisms are favorable for others. Extremes of temperature, high and low ph, high salt concentration, high levels of radiation kill the majority of organisms on Earth. However, there are organisms from all three domain of life that have adapted to these extreme conditions. Probably high temperature, low ph, high salinity persisted throughout Earth history (and icy environments). These environments are not rare on Earth today as well. There are very few natural environments on Earth where life is absent! Life is a rule rather than an exception! The limits of life are yet to be discovered.

4 Evidence towards the origin of life Old rocks have been hidden or destroyed by geological processes like burial, erosion, subduction back into the mantle. The geological record becomes incomplete as we look back in time, and no known rocks survive from Earth`s first half-billion years. Geological record cannot tell us how life originated! However, there are three lines of evidence to indicate that life arose quite early on Earth. 1. Stromatolites 2. Microfossils 3. Isotopic evidence

5 1. Stromatolites Stromatolites are layers of sediment that once contained colonies of ancient microbes. Stromatolites date back to approximately 3.5 billion years ago. They have a distinctive layered structure. Regarding size, shape, interior structure ancient stromatolites look identical to mats formed today. They also contain chemical and isotopic evidence of biological origin. Layers of sediments intermixed with microbes. Microbes at the top generate energy via photosynthesis, the microbes beneath use the organic compounds (waste) of the photosynthetic microbes. They grow over time as sediments are depositing over, forcing them to migrate upward in order to perform photosynthesis.

6 1. Stromatolites Pre-Cambrian stromatolites, Glacier National Park. In 2002 W. Schopf published a paper in the scientific journal Nature arguing that geological formations such as this possess 3.5 billion year old fossilized algae microbes, the earliest known life on earth.

7 1. Stromatolites If the microbes in the ancient stromatolites are like in the modern ones, this means that at least some produced energy by photosynthesis Photosynthesis is a sophisticated metabolic process it took a long time for this process to evolve Therefore primitive life should have already existed some long before 3.5 billion years ago!

8 2. Microfossils Second life of evidence for the early life on Earth microfossils highly controversial. Microfossil evidence suggests that life existed before about 3.0 billion years ago and may well have existed before 3.5 billion years ago. Difficulties: - Finding old fossils is very difficult the oldest rocks have been altered by geological processes destroying microfossils. When searching for evidence of past life, sedimentary environments are considered the most suitable because they are often formed in association with water, a fundamental requirement for life. There are only three known locations that host exposures of ancient sediments: Greenland, which are 3.8 to 3.7 billion years old (Ga), in Australia (3.5 to 3.3 Ga), and South Africa (3.5 to 3.3 Ga). - Contamination of ancient rocks by recent endolithic microorganisms further complicates the task of identifying the original signatures of life! - The claim of the discovery of microfossils is controversial because it is not clear whether they are of biological or mineral origin.

9 2. Microfossils Shape and organic content suggests this is a microfossil of ancient living cell dated back 3.5 billion years ago in Australia; chemical analysis shows the presence of organic carbon. The rock presumed to be a sedimentary rock from a shallow sea and the fossils of a photosynthetic microbe. However, the rocks analysis indicates is from a deep-sea volcanic vent, similar to a black smoker; therefore the fossil cannot be of a photosynthetic microbe. Microbial biofilms and mats assumed to be using a primitive form of photosynthesis (anoxygenic) - filamentous, coccoid, or rodshaped microbes have been found in sediments that formed in shallow-water basins environments, billion years old (Walsh, Precambrian Res. 54, 271, 1992; Tice and Lowe, Nature 431, 549, 2004).

10 2. Microfossils

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12 2. Microfossils

13 2. Microfossils

14 2. Abiotic microfossils?

15 Phylogenetic tree - Time scale Phylogenetic tree of life with dates indicating the minimum age of selected branches based on fossil evidence and chemical biomarkers. The length of the branches has no temporal scale - related only to evolutionary distance, not geological time! We can show some ages estimated from the fossil record. The earlies presence of eucaryotes indicated by steroids (sterane precursors - rigidify molecules within the lipid layer in the cell membrane - give ability to engulf large particles, allows endosymbiosis (living inside) of organelles.

16 3. Isotopic evidence Studies of carbon isotopes can be used to detect the presence of past biological activity in rocks. The earliest evidence for life comes from chemical remnants of biological processes in rocks. Many fundamental biochemical processes involving the extraction of carbon from an abiotic reservoir preferentially take up 12 C rather than 13 C. e.g an enzyme that captures carbon from carbon dioxide, preferentially removes 12 C from CO 2 because of the weaker carbon-oxygen bond strength in the lighter isotope. Hence, remnants of organic carbon (processed though cells) should be enriched in 12 C/ 13 C relative to the contemporaneous atmospheric CO 2. Carbon isotope evidence from rocks found in Greenland, although controversial, suggests that life may have existed 3.85 billion years ago. Ancient rock formation of the coast of Greenland. Rocks of similar age (3.8 billion years old) show similar carbon isotopic ratios to Greenland rocks. Life can also alter isotopic ratios of other elements (Fe, N, S). These isotopes are present in characteristic ratios for life within the ancient rocks, confirming the existence of life as early as 3.85 billion years ago.

17 3. Isotopic evidence Sedimentary carbon isotope values through time (middle dashed line is the mean and outer two lines are one standard deviation) Carbonate carbon Organic carbon Carbon isotope ranges of major groups of higher plants and micro-organisms compared with the respective ranges of eh principal inorganic carbon species in the environment. (Schidlowski 1983)

18 When did early life begin Life arose considerable earlier than 3.85 billion years ago. The young Earth probably experienced numerous large impacts during the heavy bombardment, some being large enough to extinguish any life, or eat least life near the surface. Conclusion: Current geological evidence suggests that life appeared very soon after the heavy bombardment in a short time period of perhaps only 200 million years. We could expect life to arise as fast on any other world with similar conditions (many other worlds with similar conditions).

19 What did early life look like The earliest living organisms presumably went extinct long ago. The current organisms most closely related to the common ancestor of all life on Earth would be located very closed to the root of the tree of life. Organisms close to each other on the Tree of Life are genetically very similar, while the ones far from each other are genetically very different. As life on Earth evolved, its DNA became gradually more complex. Present-day living organisms closest to the root of the Phylogenetic tree are hyperthermophiles. The Phylogenetic tree suggests that present day life on Earth evolved from a hyperthermophile ancestor, even though earlier life might have had greater variety (but became extinguish by a near-sterilizing impact. Universal phylogenetic tree features hyperthermophilic (grow >90 o ), and cold adapted species phychrophilic (blue lines), or psychrotolerant (violet lines) of Bacteria and Archaea

20 The melting temperature of a protein whose thermal stability is correlated with the growth temperature of the host organism, versus the geological time. Solid lines are temperature curves inferred from maximum 18 O.

21 Where did life begin 1. On land surface? NO Life probably did not originate on the land surface because there was no ozone layer to shield out harmful UV rays. (only Deinococcus Radiodurans like organisms could survive) 2. Shallow ponds? NO full of organic compounds; as the water evaporated they became more concentrated making it easier to combine into more complex molecules. However, shallow ponds would have lacked a source of chemical energy. In addition, life in ponds would have suffered lethal effect of UV radiation and or high salt concentration. 3. Deep-sea vents? Probably. DNA evidence suggest early life was hyperthermophile. Offer plenty of chemical energy to fuel metabolism, protected from the Sun`s UV rays by the water above, safe from impacts that partially vaporized the oceans during the heavy bombardment the safest environment! Universal phylogenetic tree features hyperthermophilic (grow >90 o ), and cold adapted species phychrophilic (blue lines), or psychrotolerant (violet lines) of Bacteria and Archaea

22 The origin of life Abiogenesis - Scientific consensus is that life on Earth might have emerged from non-life between 4.4 billion years ago, when water vapor first liquefied, and 3.9 billion years ago, when the ratio of stable isotopes of carbon ( 12 C and 13 C ), iron and sulfur points to a biogenic origin of minerals and sediments. Panspermia - Life could have an extraterrestrial origins of life. The most likely candidates for a source of organisms in the early Solar System are Mars and Venus. Microstructures in martian meteorite ALH84001 claimed to be of biogenic origin 1n 1996 NASA made the announcement that a martian meteorite ALH84001 contains fossils of alien micro-organisms. The meteorite Alan Hills ALH84001 has a fascinating history: 4.5 Ga crystallized from magma on Mars 4.0 Ga battered by not ejected by an asteroid impact Ga altered by water to prodice carbonate minerals 16 Ma blasted into space by an asteroid impact 1984 Discovered in Antarctica Mars Venus

23 The possibility of migration Among the 20,000 meteorites identified, chemical analysis revealed that about 30 came from Mars. These meteorites were blasted by large impacts, then followed orbital trajectories that caused them to land on Earth. During the last 4 billion years, the inner planets (Venus, Earth, Mars) have exchanged many tons of rocks - giving the possibility of microscopic life hitchhiking on meteorites! The microbes had survive - The impact that blasted the surface on its home world - Time spent in the outer space (cold, UV, radiation, cosmic rays) - The impact in the new world The interior of many meteorites shows minimal disruption, suggesting that a microbe inside the rocks could survive the two impacts. The chance of surviving the trip between planets would depend on how long the meteorite spends in the outer space. Many meteorites will orbit for many millions of years before reaching Earth, however 1 in 10,000 meteorites may travel from Mars to earth in a decade or less. We have seen that microbes can be revived after thousands of years of being frozen, therefore panspermia from a neighborong planet is possible! Panspermia from other stars systems is unlikely, because of the large distances between stars.

24 Reasons to consider migration Venus Earth Mars Why do we have any reason to suppose that life originated elsewhere rather than on Earth? 1) If life does not form easily, at least under the conditions on early Earth. Did some other world in our Solar System has better conditions for life? We have no reason to believe this. Also, they did not had more time for life to appear, because the planets formed at the same time. 2) If life forms very easily that we should expect life to originate on any planet under suitable conditions. Life occurred on whichever planet got the conditions first. In this scenario life did not originate on Earth because life from another planet got here first. Venus had once oceans and a habitable climate, however now is so hot that any fossil record would be destroyed by heat and geological activity. Conclusion: life arose on Earth quite soon after the conditions allowed it.

25 Implications of migration to the search for life beyond Earth Venus Earth Mars It is almost certain that microbes from Earth traveled to the Moon, Venus, Mercury, and Mars. If life survived on any of these worlds, we should find it there. Probably it did not survive on the Moon, Mercury, and probably Venus. Mars might have habitats that could provide a temporary refuge to terrestrial microbes. Therefore, if we find life on Mars, we will ask: is it native or it came from Earth? The biochemistry will provide the answer.

26 Organic compounds in meteorites

27 Organic compounds in interstellar space Some of the 120 compounds detected in the interstellar medium, mostly by microwave spectroscopy (courtesy Lucy Ziurys).

28 Next lecture Evolution of life. Part II. Assignment: due on March 17th Imagine you are attending a conference on Astrobiology and you have to write an abstract for the conference ( words the body of the text). The subject: any subject of astrobiology. Be creative! Think out of the box! Try to bring your own ideas! Look for controversies! Your text should have between 3 to 5 references to scientific publications. 10 of the best abstracts will have oral presentations during one of the class! The rest will be posted on the Phys 214 web.