Tópico: Is There Any Plausible Reason Why Aliens Would Evolve To Look Like Us?

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Is There Any Plausible Reason Why Aliens Would Evolve To Look Like Us?


In science fiction movies and TV shows, intelligent aliens are usually the same basic shape as humans: two arms, two legs and a head. But why would creatures that evolved on a completely different planet look so similar to us? We asked some experts, and they told us the most likely explanations for humanoid aliens.

The truth is, aliens tend to look like us in science fiction for a couple of basic reasons: budget, and relatability.

"Most aliens in SF are humanoid because humans produce SF," says Michael H. New, an Astrobiology Discipline Specialist at NASA. "While we are interested in the 'other,' our conception of otherness is often limited."

And a lot of experts firmly believe that aliens would not look at all like humans. For example, Stephen Jay Gould claims that life that evolved elsewhere would look totally different from us — and in fact, if you "reran the tape" from the beginning of life on Earth, you wouldn't end up with humans on this planet either. The emergence of humanoids on Earth is a totally random event that was a fluke, even with the exact conditions that we arose from.

But let's say that we do meet aliens, and they turn out to be bipeds with a roughly human-like shape... how do we explain that?

Panspermia

This is the most common explanation for creatures that look sort of like us turning up all over the universe. Either humanoid aliens spread their DNA across the galaxy to give rise to creatures in their image, or the DNA just spread through the galaxy on its own, on asteroids and stuff.

"I'm of the strong opinion that if humanoid aliens exist, they must have some genetic heritage in common with human beings," says Mark A. Bullock with the Southwest Research Institute. He'd find that easier to believe than the notion that humanoids could evolve independently elsewhere. Plus "it's been shown that panspermia is quite a viable mechanism, so the interchange of genetic material between worlds is not out of the question."

If the galaxy really did turn out to be full of humanoid aliens, "some kind of panspermia wouldn't be a bad explanation," New tells io9. "We're bilaterally symmetric and bipedal because our ancestors were." It's entirely possible that if certain events had played out differently, the dominant species on Earth would have had a very different shape.

The Burgess Shale, which is roughly 500 million years old, "displays a wide range of body plans, only some of which are still seen on the modern Earth," adds New. So he believes you'd need some outside intervention to account for humanoid aliens.

Bullock sounds a similar note, saying that the Cambrian explosion, 600 million years ago, "saw a great deal of evolutionary experimentation with body plans," some of which could be a glimpse of life forms that we might see on other planets.

At the same time, panspermia is only really likely at the microbial level, cautions Joan L. Slonczewski, a biology professor at Kenyon College and science fiction author whose books include A Door Into Ocean and The Highest Frontier. Beyond microbes, panspermia doesn't really make much sense as an explanation for humans' own development.

"Humans on Earth are so obviously a part of Earth's evolutionary program," says Sclonczewski. "From the molecular and cellular level, to the shape of organisms, we humans evolved here."

Convergent Evolution

Or maybe humanoids just evolved on other planets, separately from us, because they just arrived at the same destination via other paths?

There are certain things about humans that helped us rise over other primates, says James Kasting, a distinguished professor of Geoscience at Penn State University. Our opposable thumbs helped us grasp tree branches, and also hold tools. And walking upright was useful, as well. Finally, being warm-blooded helped us to power our big brains.

"I would think that there's a good chance that intelligent alien life evolved in more or less the same way and would thus bear some resemblance to humans," says Kasting. "Not necessarily a close resemblance, though."

The upright-walking, bipedal, two-armed posture "seems to have evolved independently in various unlikely contexts, from meerkats to velociraptors," notes Slonczewski. "Maybe it just makes sense to have two feet to move, two hands to manipulate something, and a sensory 'head' with as wide a view as possible. Then again, that's what we have, so it makes sense to us."

We've seen enough examples of convergent evolution on Earth to believe that it could happen on other planets as well, notes Steven J.
Dick, the 2013-2014 Baruch S. Blumberg NASA/Library of Congress Chair in Astrobiology at the Library of Congress. "For example, the eye has been reinvented many times independently, as have wings in insects, birds and bats. Fish and marine mammals such as dolphins have evolved streamlined shapes for their water environment."

Dick recommends the 1981 book Life in Darwin's Universe: Evolution and the Cosmos by Gene Bylinsky, which argues that "a limited number of engineering solutions" are possible when it comes to successful life forms.
But Dick adds that you can't discount environmental factors which would ensure that life on other planets would look at least somewhat different, including gravity. Dick tells io9:

Because they would have been shaped by their own unique planetary environments, organisms would be different from us in the particulars, just as there is great diversity of life on Earth, including the different requirements of land and water organisms. More generally, gravity imposes size limitations on life; from the cell to the whale is a large range indeed, but the food system of the whale (and the dinosaur on land) must strain to feed such a large structure, even as the heart struggles to sustain its blood flow. Life on a low-gravity planet might be free to soar upward both in the plant and animal kingdom, while life on a high-gravity planet would be correspondingly stifled.

Bilateral Symmetry

Let's say that the notion of aliens separately evolving bodies that have more or less a human silhouette is kind of unlikely — it's still possible that bilateral symmetry could be a constant among intelligent life forms, say some experts. This refers to the fact that your left and right sides are more or less the same, with an eye, an ear, an arm and a leg on either side.

"Bilateral symmetry appeared independently several different times in the evolution of larger organisms on Earth," says Bullock. "So bilateral symmetry may be a common feature of intelligent life, regardless of whether its specific body plan."

And once you get bilateral symmetry, you are going to start drifting in the direction of a vaguely humanoid body plan, argues Bjørn Østman with Michigan State University. The symmetry means you'll have an even number of limbs — which is most likely going to be four, rather than six or more, which don't convey enough of an advantage to justify the extra limbs.

"Even on earth there are lots of animals that have more than two pairs of limbs," concedes Østman. "But I think that the reason why we have lots and lots of animals that hva four limbs is that that's highly advantageous. It just happens to be mechanically a very good solution to traversing a rugged landscape."

And once you have a lot of quadripeds on land, one of those quadripeds is going to start using its front limbs to manipulate tools. "If you can free two limbs to manipulate tools, then it becomes very advantageous to develop high intelligence," notes Østman.

So assuming an intelligent alien is symmetrical and has some of its limbs devoted to tool use, then it might end up being roughly bipedal, says Østman. And the sensory organs, like eyes, will have to be forward-looking and not too far away from the tool-using limbs. Which means you end up with something like a head, because the nervous system will be close to the sensory organs for maximum efficiency.

Thus those two factors — symmetry and tool use — may lend themselves to something at least vaguely similar to a human shape, in Østman's view.

"If we were to eventually find other intelligent life in the universe, they would be humanoid, I think," Østman concludes. "I find that a high probability." But at the same time, he admits he's in the minority, and most other scientists agree with Gould that humanoid life is unlikely to evolve elsewhere.

http://io9.com/is-there-any-plausible-reason-why-aliens-would-evolve-t-1638235680/+megneal

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Could aliens have created life on Earth?

We know a lot about the history of life on Earth, but how it began is still one of our greatest scientific mysteries. One hypothesis is that life actually originated on another planet, and many scientists today take the idea quite seriously. Though it sounds like the plot from recent scifi movie Prometheus, it's an old idea that even the celebrated nineteenth century physicist Lord Kelvin and Nobel winning geneticist Francis Crick have advocated. That's right — the evolution of life might have its beginnings on another planet.

Over 120 years ago, Kelvin shocked the British scientific community in a speech about what he called "panspermia," where he suggested that life might have come from planets smashing into each other and sending bits of life hurtling through space. He and a few colleagues had hit upon this notion after observing the massive 1880 eruption of a volcano on Krakatoa. To be more precise, they observed the aftermath of the volcano, which completely sterilized the island. No life was left at all. But then, within months, seedlings began to sprout and life took hold again.

Where had that life come from? To naturalists of the nineteenth century, it was obvious that it had drifted there from nearby islands. Seeds and insects blown on the wind, or floating on the tides, had begun the process of re-greening the stricken landscape. This got Kelvin thinking about the origin of life on Earth. Couldn't the same thing happen to barren planets drifting in space? Perhaps life had drifted to Earth on the stellar winds.

Aliens Seeded the Planet with Life

Today, we know that most life wouldn't survive the trip through space. It would be bombarded by radiation and subjected to hard vacuum. But Francis Crick, who was one of the first biologists to identify the structure of DNA, suggested a way around this problem. In a 1972 paper he co-authored with biologist Leslie Orgel called "Directed Panspermia," Crick suggested that perhaps extraterrestrials had seeded the Earth with microbes sent in specialized spaceships that would protect the microbes. This is an idea we see a lot in science fiction, including Prometheus. Still, Crick and Orgel didn't imagine aliens dribbling DNA into our water supply — they suggested it might have been sent out in automated probes, perhaps with a kind of "missionary zeal."

The problem, which Crick and Orgel discuss in the paper, is that it's incredibly hard to prove this hypothesis, or even to gather evidence one way or the other. That's why most scientists who study panspermia don't have much to say about the directed panspermia scenario. "It's not completely ridiculous," Purdue geophysicist Jay Melosh told io9. "It's fun to speculate about, but it's not the subject of really respectable scientific research because there's no evidence."

That said, Melosh and many other scientists do think panspermia might be part of the solution to the mystery of how life began.

We Come from Mars (or Europa)

Directed panspermia is simply the most unlikely version of a story that is actually quite plausible. Take out the aliens and the spaceships, and you still have many possible ways that microbes from other worlds might have made it to Earth. And if those microbes came from nearby, the panspermia scenario becomes even more plausible.

Cal Tech geologist Joe Kirschvink has suggested that Mars is a likely origin for life in the solar system because it would have been habitable long before Earth was. 4 billion years ago, when Earth was still a roiling cauldron of methane and magma, Mars was a stable, cool planet covered in vast oceans. It would have been the perfect place for microbial life to take hold. But how did that life make it all the way from the seas of Mars to the seas of Earth? Most likely, meteorites crashing into Mars would send fragments of the planet's surface back into space — packed with millions of microbes. In fact, around the time that Mars might have been developing life, the solar system was undergoing what astronomers call the "late heavy bombardment," a time of countless intense meteorite strikes.

Purdue geologist Melosh, who has spent most of his career studying meteorite impacts, has actually done experiments where he and a team recreated what might have happened when meteorites slammed into Mars billions of years ago, sending ejecta out of the atmosphere and eventually all the way to Earth. This process is sometimes called "ballistic panspermia," or "lithopanspermia," because it depends on rocks being ejected into space. To recreate one part of this process in their experiments, Melosh and his team shot a bacteria-covered rock with an aluminum projectile moving at 5.4 km per second, and the shattered chunks flew over a kilometer. The bacteria survived the trauma of what Melosh and his team called "extremes of compressional shock, heating, and acceleration."

After several of these tests, Melosh and his colleagues were certain that microbes could survive one of the most destructive parts of the ballistic panspermia journey:

A lot [of the microbes] would die, but a lot would survive in a dormant state. Their journey would take possibly millions of years. But it's as if atmospheres are almost designed for this transfer of life. The meteorite comes from Mars, full of microbes protected from radiation by the rock. It enters Earth's atmosphere, and as it comes in at high speed the outside melts because of friction and gets hot, but the inside is protected just like a spacecraft capsule. The microbes inside are protected. Then the aerodynamic forces in the lower atmosphere fracture the meteorite, exposing the interior.

The rock fragments rain over the land, and the surviving microbes can take hold.

Most scientists who subscribe to this idea suggest that Mars is the likely source for a ballistic panspermia event, though Melosh isn't ruling out Jupiter's moon Europa either. Astronomers believe Europa harbors vast oceans beneath a thick layer of ice, and it's very possible that a meteorite could have crashed there, sending microbe-laced chunks of rocky ice into the inner solar system. Still other scientists suggest that life could even jump from one star system to another, and a recent paper on the topic explores how this could happen in star clusters. Still, it's not likely that Earth was seeded from beyond the system — unless aliens were behind it.


A Valid Scientific Hypothesis

A big question is why scientists are entertaining this idea at all. Doesn't it seem outlandish? Perhaps, but then again so is the sudden appearance of life on Earth over 3.5 billion years ago. How did we go from lifeless puddles of chemicals to strings of self-reproducing DNA on a planet that was at the time so inhospitable?

Panspermia doesn't answer this question — we still aren't sure how the life switch got flipped — but it could help explain the conditions where that life evolved.

NASA planetary scientist Chris McKay offered io9 a terrific, point-by-point explanation of why panspermia is, as he put it, "a valid scientific hypothesis" worth taking seriously:

1. The geological evidence for the earliest life on Earth is very early, soon after the end of the late bombardment. There is good evidence for life on Earth at 3.5 billion years ago, indirect evidence at 3.8 billion. The end of the late heavy bombardment is 3.8 billion years ago.

2. The genetic evidence indicates that the last universal common ancestor (LUCA) of life could have been roughly 3.5 billion years ago (but with large uncertainties) and that LUCA was a fairly sophisticated life form in terms of metabolic and genetic capabilities.

1 and 2 together give the impression that life appeared on Earth soon after the formation of suitable environments and it appears to have come in being remarkably developed - like Athena born fully formed from the head of Zeus.

3. Rocks from Mars have traveled to Earth and the internal temperatures experienced in these rocks during this trip would not have sterilized the interiors. Thus in principle life can be carried from Mars to Earth.

4. Mars did not suffer the large Moon-forming impact that would have been detrimental to the early development of life on Earth.

3 and 4 have lead to the suggestion that Mars would have been a better place for life to start in the early Solar System and it could have then been carried to Earth via meteorites.

5. Organic molecules are widespread in comets, asteroids, and the interstellar medium.

6. Comets could have supported subsurface liquid water environments soon after their formation due to internal heating by decay of radioactive aluminum.

7. As comets move past the Earth they shed dust which settles into Earth's atmosphere.

5, 6 and 7 have lead to the suggestion that life could have started in the interstellar medium or in small bodies such as comets and then been carried to the Earth by comet dust.

So, yes panspermia is a valid scientific hypotheses and warrants further investigation.

We Are Not Alone

And as we engage in that investigation, maybe we'll discover more than we bargained for. After all, if we owe our existence to life on other planets in our solar system, that makes a strong case for life outside it too. SETI astronomer Jill Tarter told io9 via email:

I think that intelligent life here on Earth is a proof of concept that it could exist elsewhere, but we will not know unless we search systematically and exhaustively enough to accumulate sufficient information to justify significant null result. Remember the last sentence of the 1959 Cocconi and Morrison paper [published in Nature]: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."

http://io9.com/5918189/could-panspermia-have-created-life-on-earth

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MARS, PANSPERMIA, AND THE ORIGIN OF LIFE:
WHERE DID IT ALL BEGIN?


During the nineteenth century, when steady-state cosmological theories were in vogue, Lord Kelvin, Svante Arrhenius, and other eminent scientists believed that the transfer of life from one planet to another was a process made inevitable by the infinite extent and duration of the universe. This hypothesis, known as panspermia, subsequently fell out of favor, partly as a result of the acceptance of the Big Bang theory. Most efforts to understand the origin of life have since been framed by the assumption that life began on Earth.

However, in the last decade data have begun to accumulate suggesting that panspermia may in fact be a natural and frequently occurring process. Recent paleomagnetic studies on Martian meteorite ALH84001 have shown that this rock traveled from Mars to Earth without its interior becoming warmer than 40ºC (Weiss et al. 2000) (Fig. 1). Experiments aboard the European Space Agency’s Long Duration Exposure Facility indicate that bacterial spores can survive in deep space for more than five years (Horneck et al. 1994; Horneck 1999), and laboratory experiments demonstrate that bacteria can survive the shocks and jerks expected for a rock ejected from Mars (Mastrapa et al. 2001). Finally, dynamical studies indicate that the transfer of rocks from Mars to Earth (and to a limited extent, vice versa) can proceed on a biologically short time scale, making it likely that organic hitchhikers have traveled between these planets many times during the history of the Solar System (Mileikowsky et al. 2000; Weiss and Kirschvink 2000). These studies demand a re-evaluation of the long-held assumption that terrestrial life evolved in isolation on Earth.

Three lines of evidence have been proposed to support the idea that by 4.0 billion years ago (Ga), life somewhere had evolved to a fairly high level of complexity. These challenge the assumption that the late heavy bombardment (Cohen et al. 2000) was inimical to the origin and continuity of life. First, the presence of isotopically depleted graphite inclusions in apatite crystals in Archean rocks from Greenland (Mojzsis et al. 1996) were interpreted as the action of biological carbon fractionation, perhaps through photosynthesis. However, this has been cast in doubt because the apatite has U/Pb and Pb/Pb ages of ~1.5 Ga, indicating either that they formed much later than the BIFs and/or their isotopic ratios were reset by a high-temperature metamorphic event (Sano et al. 1999). The geological context of the sampling area has also been questioned (Myers and Crowley 2000).

The second line of evidence is the possible presence of 3.9-4.1 Ga magnetofossils (indistinguishable from those made by modern Earthly bacteria) in the ALH84001 carbonates (Friedmann et al. 2001; Thomas-Keprta et al. 2000, 2001). Although this hypothesis remains highly controversial, no one has yet documented a plausible, inorganic mechanism for producing similar particles. The ferrite industry (which makes small magnetic particles for disk drives and magnetic recording tapes, and is worth $35 billion per year) has tried to make similar particles of magnetite inorganically for the past 60 years and has failed. The inorganic synthesis of magnetosome-like magnetite particles is clearly not a simple feat. Although Golden et al. (2001) claim to have produced magnetofossil-like magnetites from heating of siderite, they have not yet documented the shape and chemistry of those inorganic magnetites in detail (see Thomas-Keprta et al. 2001). Nevertheless, their mechanism should be studied further given that shocks have probably affected ALH84001 carbonates.

The third line of evidence is recent molecular-clock analyses which exploit the large number of completely sequenced genomes of Bacteria, Archaea, and Eukarya, and indicate that the Last Common Ancestor (LCA) of all living organisms dates back to about 4 Ga (Hedges et al. 2001) (Fig. 2). This phylogeny (Hedges et al. 2001) is more pleasing than earlier attempts using fewer gene systems because it intrinsically agrees with two deep-time aspects of the direct geological record: it agrees with the presence of the first biomarkers (2-methylhopanoids) possibly indicative of the cyanobacteria at around 2.5 Ga (Summons et al. 1999), and with the direct fossil record of organelle-bearing eukaryotes at about 2.1 Ga (Han and Runnegar 1992). Hence, it is probably the Hadean of Earth, or perhaps even the Noachian of Mars, in which we must look for Darwin’s "warm little pond".

Because life on Earth depends on a variety of biochemical respiratory chains involving electron transport, we argue that the presence of electrochemical gradients is one of the most critical prerequisites for life’s initiation and would form intense selection pressures during its initial evolution. A recent genetic analysis of the respiratory chains in the Archaea, Eukarya and Bacteria (Castresana and Moreira 1999) indicates that terminal oxidases linked to oxygen, nitrate, sulfate, and sulfur were all present in the last common ancestor of living organisms. An alternative interpretation, massive lateral gene transfer (Doolittle 2000), can be ruled out at least for O2 because most of the cytochrome oxidase genes (with the exception of a small domain in one organism) track the phylogeny of rRNA (Schutz et al. 2000). Figure 3 shows the typical electrochemical cascades that operate in a modern, neutral oceanic system, with the basal oxidase of the LCA indicated in red. Our LCA must have evolved on a world in which these redox gradients were present in large enough quantities to evolve and to be maintained, and in which metastable, energetic compounds could diffuse across redox boundaries. "Smothered" environments (like Europa’s ocean; Gaidos et al. 1999) that lack significant electrochemical gradients are difficult for existing life to survive in, and would have been even more difficult for it to arise in. Since the efficiency of biological systems tends to improve over time as a result of the processes of random mutation and natural selection (Darwin 1859), primitive metabolic electron transport systems likely could not extract energy as efficiently from the redox couples shown in Fig. 3 as do those of modern organisms. In fact, many of the intermediate steps in these electron transport chains arose through gene duplication events from ancestral proteins (Schutz et al. 2000). Hence, the ancestral forms could not have been as efficient at recovering metabolic energy as are the modern ones. For this reason, we argue that life is more likely to have evolved on the planet (either Earth or Mars) that had the broadest dynamical range of electrochemical species early in its history.

So now it is important to compare the probable environments of the early Earth with that of early Mars in order to evaluate which of these two bodies, during the first half-billion years of the solar system, might have produced an environment most suitable for the origin of life based on redox chemistry. This involves deducing the early environments of Mars and of Earth during a time interval when Earth lacks an extensive sedimentary rock record and when the Martian record must be accessed indirectly.

Likewise, the principal gaseous components produced by volcanic emissions likely had equilibrated with the reducing components then present in the outer mantles of these planetary bodies. A recent study of Martian meteorites (Wadhwa, 2001) has shown that the mantle of Mars is more reduced than that of the Earth, while the crust of Mars is still quite oxidized. The difference between the two planets is probably a result of the fact that Mars lacks plate tectonics, which on Earth recycles oxidized crust back into the mantle. However, on Earth, plate tectonics did not necessarily oxidize the mantle quickly. Kump et al. (2001) argue that during Archean time subducted oceanic crust may well have penetrated to the lower mantle, implying that the bulk of volcanic gasses equilibrated with the more primitive and more reducing upper mantle, until a turn-over event near the Archean/Proterozoic boundary. Both planets, then, possessed at least local environments sufficiently reducing to allow the accumulation of pre-biotic compounds. The question then focuses on determining which planet would have contained a better source of oxidizing atmospheric compounds capable of diffusing into this primordial soup to promote organic evolution.

Figure 2 (adapted from Kasting 1993) shows the redox history of Earth’s surface, as inferred through the geological record. Although today both Earth and Mars have extremely oxidizing surficial environments, there has been a long debate about the redox potential of the Earth's surface prior to the Phanerozoic (summarized in Schopf and Klein 1992). Sedimentary rocks of Archean and early Proterozoic age typically contain banded iron formations (BIFs) and detrital, stream-rounded pebbles of the minerals pyrite and uraninite. Based on modern analogs, these are usually taken as indicative of more reducing conditions. In contrast, the strongest line of evidence for oxygen production in the Archean was the presence of filaments with strong similarities to cyanobacteria from black cherts in the 3.5 Ga Warawoona group of Australia (Schopf 1993; Schopf and Packer 1987). These claims supported an environmental model of small oxygenic ‘islands’ of photosynthetic life separated by sharp redox gradients from an otherwise anaerobic environment (e.g., Cloud 1988). However, Buick (1988) has maintained that the actual black cherts sampled for these studies were secondary hydrothermal silica deposits of much younger age which cross-cut the primary bedding (an observation with which we concur after having visited these localities with him this past July). These putative fossils are probably not a constraint on oxygen in the Archean environment. Furthermore, it is unclear why the cyanobacteria, if equipped with the incredible power of the oxygen-releasing Photosystem-II, did not follow an explosive exponential growth pattern to take over the world as soon as they evolved. Even the presence of Archean stromatolites does not help, given their possible abiogenicity (Grotzinger and Knoll 1999) and the possibility that their photosystems were non-oxygenic. In fact, the oldest direct evidence for oxygenic photosynthesis (Photosystem-II) is the Kalahari Manganese Field, which precipitated as oxides in the aftermath of the Paleoproterozoic Snowball glaciation event at about 2.3 Ga (Kirschvink et al. 2000). These deposits could well be the result of run-away oxygenation of the surface environment as a result of the newly evolved Photosystem-II. As shown on Fig. 2, the Kalahari Manganese Field falls well within the error range for the evolution of cyanobacteria given by the Hedges et al. (2001) molecular-clock study, but it is clearly out of range for the LCA.

In recent years, several new developments have bolstered the case for a strongly reducing surficial environment on Earth during Archean and earliest Proterozoic time. First, shallow-water carbonates of this age typically contain a trace of Mn+2, which is only soluble under anoxic conditions (Kirschvink et al. 2000; Veizer 1994), requiring oxidants of either nitrate or O2 to be oxidized to the insoluble Mn+4 form (see Fig. 3). But nitrate formation in the ocean requires O2 (Kirschvink et al. 2000), so this is de facto a constraint on O2. The Mn+2 presumably was incorporated in the carbonate during initial precipitation, but it disappears after the first Snowball event at 2.3 Ga. This implies that post-Snowball oxygen levels were high enough to keep Mn+2 out of solution in the surficial waters. Second, Sumner (1997) has noted that Fe+2 has a strong inhibitory effect on the nucleation of carbonate minerals, and that many of the shallow-water early Archean carbonates display textures compatible with the presence of ferrous iron in the shallow, mixed surface waters.

Third, recent work on terrestrial mass-independent sulfur isotope fractionation (Farquhar et al. 2000) suggests that units older than the Snowball were derived from an environment reducing enough to allow gaseous sulfur compounds to be cycled within the atmosphere. The implications of sulfur fractionations for the early Martian redox environment are less clear. Although ~4 Ga ALH84001 pyrites also show enormous mass-independent sulfur fractionations, these do not require a reducing atmosphere but instead can be explained by the lack of crustal recycling on Mars, in combination with an early influx of isotopically heterogeneous materials to the planet (Greenwood et al. 2000). Furthermore, mass-independent fractionation of oxygen isotopes (17O with reference to 16O and 18O) in ALH84001 carbonates suggest passage of this element through an atmospheric ozone phase (Farquhar et al. 1998).

Fourth, the rarity of glacial features during Archean time, coupled with solar evolution models arguing for a faint-young-sun with 30% less luminosity than today, has long presented a climatic paradox in its own right. Suggestions of a massive, 10-bar CO2 greenhouse atmosphere (Kasting 1993) conflict with lower estimates of atmospheric CO2 from Archean paleosols (Rye et al. 1995). However, Pavlov et al. (2000) note that an Archean atmosphere rich in methane would solve this paradox, and would provide an elegant trigger for the Paleoproterozoic Snowball event as soon as Photosystem-II were to evolve. If these indicators are extrapolated blindly back to the Hadean Earth, environments capable of producing sharp redox gradients needed for the evolution of primitive life would be rare to nonexistent.

The temporal evolution of oxygen in the Martian atmosphere is much less understood. In the present-day atmosphere, H and O atoms are thought to be lost from the atmosphere to space in a self-regulating ratio of 2:1, which maintains a constant oxidation state for the planet. This loss ratio is maintained today because the rates of formation and photolysis of H2O adjust to maintain the appropriate supplies of free H and O available for loss (Yung and DeMore 1999). The constant 2:1 loss ratio is maintained despite the fact that the principal loss mechanisms for these two elements are vastly different. Hydrogen is lost mainly via thermal escape (Nair et al. 1994). On the other hand, O, a much heavier element, is lost chiefly from the impingement of the solar wind plasma and magnetic field (e.g., atmospheric sputtering, dissociative recombination, and direct ion pick-up; Fox 1997; Hutchins et al. 1997). Both hydrogen and oxygen loss have had a much less significant effect on Earth’s atmosphere both because of Earth’s higher gravity and strong magnetic field (the latter reduces solar-wind-induced ionization as well as loss of ionized species due to interaction with the solar-wind magnetic field).

Although Mars currently lacks a global magnetic field, we have recently learned that it had a strong field at 4 Ga or earlier (Acuna et al. 1999, Weiss, B. P., Vali, H., Baudenbacher, F. J., Stewart, S. T., and Kirschvink, J. L., unpublished manuscript). The ancient field would have protected against the loss of O (Jakosky and Phillips 2001). If today’s self-regulating loss mechanisms for H and O also operated in this early period in Mars history, this would imply that H loss would have also have been much less in this period. However, there are good reasons to believe that this was not the case. The D/H ratio of the present-day atmosphere is ~5 times that of the Earth, indicating that a large amount of H (and, by implication, H2O) has been lost to space. However, the 18O/16O ratio is not much different than the terrestrial value, indicating that O and H loss were decoupled in the past (Owen 1992). This means that the protection afforded by the early magnetic field might have meant dramatically more loss of H than O, and so provided a cascade of oxidants to drive organic evolution. Given that Mars's mantle was likely more reduced than that of Earth (Wadhwa, 2001), such oxidizing conditions at the surface would imply that Mars had much larger redox gradients than Earth at the same time.

Isotopic studies of the Martian meteorite ALH84001 are consistent with a neutral to oxidizing surface environment in which the carbonates formed about 4 billion years ago, including contributions from photo-irradiated ozone (Farquhar et al. 1998). This ozone would have shielded an early Martian biosphere from UV radiation, a protection presumably lacking on Earth at the same time (Pavlov et al. 2001). Also, the possible presence of magnetofossils in this meteorite argues for the presence of vertical redox gradients, which magnetotactic bacteria need for their survival (Chang and Kirschvink 1989). The oldest magnetofossils yet identified on Earth are from the post-Snowball Gunflint Chert at about 2.1 Ga (Chang and Kirschvink 1989), roughly coincident with the appearance of the first eukaryotes (Han and Runnegar 1992).

At face value, all of these lines of evidence suggest that, compared to early Earth, early Mars might have had a greater supply of biologically useable energy and was perhaps, by implication, a better place for the origin of life. And so we salute you, all you descendants of space-traveling microbes from the Red Planet!

Acknowledgments

We thank F. Macdonald and F. Baudenbacher for assembling the image of Figure 1, and the NASA Astrobiology Institute and the NASA Exobiology program for supporting this research.

This text is a modified version of the Carl Sagan Lecture given by Joseph Kirschvink at the American Geophysical Union Meeting in December, 2001 (webcast of lecture here).

Joseph L. Kirschvink. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125, USA
and the Department of Earth and Planetary Sciences, Tokyo University, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Benjamin P. Weiss. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena CA 91125, USA

http://palaeo-electronica.org/2001_2/editor/mars.htm

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http://en.wikipedia.org/wiki/Convergent_evolution

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<a href="http://www.youtube.com/v/k-j69iVReEU#t=72" target="_blank" class="new_win">http://www.youtube.com/v/k-j69iVReEU#t=72</a>

[Buckaroo Banzai]

Segundo as idéias de que o universo é infinito, não só haveria uma grande infinidade de seres "meio" como nós, humanóides, ETs de cinema, mas também uma infinidade, algo menor, de seres exatamente como nós. Até mesmo individualmente.

Asimov ou alguém que o valha já colocou algo assim como uma espécie de "reencarnação" 100% materialista.