Hallo Beisammen,
Es gab da gestern einen interessanten online Vortrag von David Deamer und Bruce Damer beim SETI Institut. Der war bei Zoom zu sehen, eine Aufzeichnung bei Youtube ist leider nocht nicht da. Wenn da was kommt, bitte angucken.
Beide haben ein Paper: "The Hot Spring Hypothesis for an Origin of Life"
https://www.liebertpub.com/doi/10.1089/ast.2019.2045
In dem Vortrag haben sie das noch weiter ausgearbeitet und kamen zu einem Punkt, dass die Entstehung von Leben unwahrscheinlich ist (und damit z.B. Fermi Paradoxon beantwortet)
- Sie meinen Entstehung von Leben im Meerwasser ist nicht machbar. Die Salz Ionen machen das Wasser zu hart für Membranen. Und es gibt keinen naß/trocken Zyklus, den sie für ntwendig erachten.
- Es bleibt Süßwasser an Land. Dazu muss es Land geben, also Kontinente. Keine Waterworld. Sowas wie Enceladus / Europa mag heiße Tiefseequellen haben, aber da passiert nix.
- In dem Vortrag wurde auch die richtige Anreicherung von U & Th in der Erde als notwedig angegeben (Gerade die Richtige Entfernung zu einem kollidierenden Neutronenstern-Paar zum präsolaren Nebel, um diese Elemente zeitgerecht zu produzieren). Es braucht genug (aber nicht zuviel) Wärme im Planeten-Kern, um Vulkanismus und Plattentektonik zu ermöglichen.
- Ihre Theorie basiert auf warmen Tümpeln (Geysir), die zyklisch austrocknenen und nass werden. Das braucht es um aus Membranen Kügelchen zu bilden in denen immer neue Bauelemente gemischt werden, bis Evolution übernehmen kann. Es braucht den Zyklus von eintrocknen / in Süßwasser auflösen für sehr lange Zeit bis biologische Zellen entstehen.
Im Vortrag sind sie zum Schluss gekommen, dass die Entstehung von Leben (Faktor in Drake-Gleichung) eher unwahrscheinlich ist. Es mag einen Haufen von Planeten mit flüssigem Wasser geben, aber die genaue Kombination von Land, Vulkanismus und Süßwasser ist unwahrscheinlich.
Hier die letzten zwei Kapitel aus dem Paper.
8.2. Astrobiology implications of the hypothesis for the origin of life on other worlds
Continuously submerged environments such as hydrothermal vents clearly support highly adapted forms of life on Earth today and might possibly harbor life on ocean worlds such as the icy moons Europa and Enceladus (Fig. 10, left). Plumes containing organic compounds and water emerging through cracks in Enceladus' icy shell were discovered by the Cassini mission (McKay et al., 2008), suggesting that hydrothermal vents might be operating at a rock interface deep in an interior ocean. However, while potentially habitable, due to their submarine setting and lack of wet-dry cycling, these locations may not possess the chemical, thermodynamic, or combinatorial capacity for life to begin (Deamer and Damer, 2017). Mars, on the contrary, has clear evidence of silica deposits (Fig. 10, right), indicative of subaerial hydrothermal systems of a similar age as those discovered in the Pilbara. Surprisingly, this site at Columbia Hills discovered by the Spirit rover also resembles hot spring environments that preserve biosignatures of microbial communities on Earth (Ruff and Farmer, 2016). This discovery can guide future missions to promising places to search for signs of past life on Mars before it lost its surface habitability.
8.3. Concluding remarks
The hot spring hypothesis for an origin of life emerged from a fortuitous collaboration across multiple branches of science. Laboratory demonstrations supporting the “RNA world” hypothesis since the 1980s established an exciting direction in our field (Gilbert, 1986). However, despite the surprising catalytic properties of RNA molecules, we contend that the chemistry of life's beginning cannot be reduced to the action of a single molecular species, but instead required the interactions and selection of vast numbers of encapsulated polymer systems. These protocells were initially formed through processes of self-assembly of both the polymers and their compartments and evolved through the engine of a kinetic trap subjecting them to combinatorial selection within cycling pools. This pre-Darwinian system of selection provided a scaffold for the “booting up” of the active molecular systems of life. Metaphors and insights from computer science have also informed the hypothesis by predicting that a collaborative network effect would occur in aggregates of protocells known as progenotes. Experimental work at multiple laboratories and more recently at field analog sites has accumulated a growing body of evidence. Finally, the discovery of 3.5-billion-year-old Archean fossil hot spring stromatolites of the Pilbara region of Western Australia provided key additional pieces to the puzzle.
It possibly required more than half a billion years for the first fragile protocells emerging in hydrothermal field pools to evolve into the robust microbial life that produced the durable fossil evidence of stromatolites. Somewhere along that timeline, the LUCAs appeared with a full complement of the functional polymers of life as we know it today. Note the purposeful use of plural in the previous sentence to indicate that LUCA was not a single cell from which all subsequent life descended, but instead would have been collectively sourced from widespread microbial mat communities that were sharing genetic information. Orgel (1968) and Morowitz (1992) proposed that all living systems follow a principle of continuity in which new structures and systems emerge through variations on similar earlier forms. A final and perhaps the most remarkable implication of this hypothesis is that the earliest form life would have taken was an aggregate of collaborating units rather than competing individuals. Peering back through the lens of the principle of continuity, stromatolites are clearly the work of microbial mat communities. Perhaps those communities trace their ancestry to a simpler form of community, the progenote, which in turn got its start in deeper time as an aggregate of even simpler self-assembled protocells with encapsulated molecular systems. The tangled roots of the tree of life traced by intertwined protocell and progenote evolution may never be unraveled to reveal a clear picture of how life actually started on the Earth. However, the hypothesis proposed here can be tested at several stages in the laboratory and in the field. An accumulating weight of evidence may then lead to understanding how life can start on a habitable planet such as the early Earth.
Clear Skies,
Gert
