The vast changes our planet has undergone since formation add a real sense of humility to the exoplanet hunt. It’s the humility that comes with exposure to deep time, reminding us that worlds like ours have developed through phases wildly different than the conditions we experience today. As we tune up our techniques for studying rocky worlds, we’ll find planets in entirely different states of their own evolution, perhaps some with life, and some young enough to be life’s future home. Perhaps some will be worlds where life has come, and gone.
We have much to learn about how our own planet developed life, and new work from an international team led by Philip Pogge von Strandmann (University College London) gives us insight into a key issue: How long did it take for oxygen levels in the oceans and atmosphere to increase to the point that animal life could take off some 600 million years ago? Underlying the question is how the evolution of life ties in to changes in climate. For it seems likely that increased oxygen was the factor that brought about the first major expansion of animal life.
Co-author David Catling (University of Washington) defines the issue this way:
“Oxygen was like a slow fuse to the explosion of animal life. Around 635 Ma, enough oxygen probably existed to support tiny sponges. Then, after 580 Ma, strange creatures, as thin as crêpes, lived on a lightly oxygenated seafloor. Fifty million years later, vertebrate ancestors were gliding through oxygen-rich seawater. Tracking how oxygen increased is the first step towards understanding why it took so long. Ultimately, a grasp of geologic controls on oxygen levels can help us understand whether animal-like life might exist or not on Earth-like planets elsewhere.”
Image: Artist’s conception of a ‘snowball Earth.’ Credit: Neethis / Wikimedia Commons.
The researchers measured selenium isotopes in rock samples laid down under the ocean to track oxygen levels from 770 to 525 million years ago (across the full period believed to be covered by what is known as the Neoproterozoic Oxygenation Event). The marine shales are drawn from seven different geological sections, with samples from Canada (the Mackenzie Mountains), China (the so-called Yangtze Platform), Australia and the western US.
This is a fascinating period because three major glaciation events occurred during it: The Sturtian (‘snowball Earth’) of roughly 716 million years ago, the Marinoan (635 million years ago, or 635Ma), and the Gaskiers glaciation (about 580 Ma).
This would be a planet that looked nothing like what we’re familiar with, as the image above suggests. The land would have been covered in ice and the oceans frozen all the way to the equatorial regions. Temperature changes as these eras progressed would in each case melt the glaciers and produce a flow of nutrients into the oceans, one that would build the levels of organic carbon in seafloor sediments as oceanic plankton that flourished from that flow died. The growth in such carbon would lead to gradual increases in the level of oxygen.
A key finding of the paper is that rather than occurring after the Gaskiers glaciation, the oxygenation began much earlier, during or at the end of the Marinoan glaciation. From the paper:
“…the significance of the Se isotope record is not only that it adds to growing evidence that the late Proterozoic and Cambrian ocean and atmosphere reached a progressively more oxic state, coinciding with the diversification of animal life, but also that the process of oxidation was protracted, and not ultimately triggered by the Gaskiers deglaciation, as other data suggest.”
Moreover, it took approximately 100 million years for atmospheric oxygen to climb from less than 1 percent to over ten percent of today’s level. The occurrence of oxygenation in fits and starts over such a lengthy period of time is evidence, the team believes, that early animal evolution received its needed boost from these increased levels of oxygen. Says Pogge von Strandmann: “We were surprised to see how long it took Earth to produce oxygen and our findings dispel theories that it was a quick process caused by a change in animal behaviour.”
The paper is von Strandmann et al., “Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere,” Nature Communications 6, article number 10157, published online 18 December 2015 (full text). A UCL news release is also available.
Si el oxigeno fue un elemento determinante para que exista vida en nuestros oceanos, ¿En que momento y como se formo la capa de ozono? Porque tengo entendido que sin este elemento no hubiese existido vida en la superficie de la tierra.
PG translation:
If oxygen was a key for life to exist in our oceans, at what point and how was the ozone layer formed? Because I have understood that without this element no life would have existed on the surface of the earth.
I think the authors are claiming too much.
1. Most theories based on other isotopic analysis indicate that the GOE wads very protracted, possibly as long as 2 bn years when cyanobacteria evolved. The O2 was kept low by the need to oxygenate Fe++.
2. Their data shows wide variation in values at each time period, suggesting that the isotopic ratio may be influenced by a number of factors beyond O2 concentration.
I see this paper as supporting the current preferred hypotheses for the GOE, rather than changing anything.
What I think is important is to note that the earth has only had an O2 atmosphere for about 1/5th of its history, while life was extant for at least 2/3rds. Therefore looking for O2 in exoplanet atmospheres is really looking for life analogous to terrestrial multicellular life, possibly even land living forms. Thus we may miss identifying many living planets because we cannot detect high levels of atmospheric O2.
While large terrestrial life forms are all aerobes (at least for most of their metabolic needs), it is possible that relatively sessile forms could evolve as anaerobes had the atmospheric O2 never have risen.
This is anaerobic versus aerobic obviously one the biggest filters limiting multi celled complexity.
Looking at data on the net:
Anaerobic respiration can turn one molecule of glucose into 2 ATP molecules as energy molecules with pyruvate as a waste product.
Aerobic respitration can convert all of the gluecose into 38 ATP molecules with CO2 and H2O as waste, though it does so at a slower rate than anaerobic path. The ability to extract so much more ATP from a single molecule of gluecose is boggles the mind.
As examples anaerobic respiration. Nitrate Respiration Or Sulfuric respiration while viable and ongoing on the earth, are suitable only in narrow biomes, and do not compete with aerobic organisms.
Pretty obvious that any primitive multicelled organism that evolved to take advantage of this would flourish and radiate many forms, due to the new energy available to experiment with. While other compounds could be used
as a substitute, none are as efficient or as abundant in the universe.
All of these only apply to ATP type electron transport type of respiration.
Nature may have come up with other paths, so I am not ready to say
that O2 is king of all. But On worlds with similar chemical abundances as
Earth, I think we will find O2 used this way by ATP analogues.
The levels of O2 seem to dictate the level of development in terms of
complexity. Is it safe to say that a world with 5% O2 may have complex life however a world with 10% or more oxygen MUST have complex life
Another nice reminder of the disconcerting fact that for ~ 90% of the history of our planet there were no large complex life forms to impress any potential tourists
Alex & Rob: Oxygen is indeed not everything. There are large terrestrial lifeforms that produce rather than consume oxygen. They do for land what cyanobacteria do for the sea. They are called trees. They (or something like them) positively could exist in a world that never had free oxygen.
marlene: The importance of the ozone layer has been greatly exaggerated. Certainly it is of insignificant consequence compared with the presence or absence of oxygen. You are right, of course, that there could not have been an ozone layer without oxygen. Most likely the atmosphere was a lot denser back then, and rich in CO2.
@RobFlores I don’t think we can presume that evolution will find the necessary solutions to fix carbon and release O2 through photosynthesis, nor that aerobic metabolism will evolve. However it is an interesting hypothesis that if photosynthesis evolves, and successfully oxygenates the atmosphere, that aerobic metabolism will evolve too. The connection between photosynthesis and respiration in cyanobacteria suggest that this may be a common connection that evolution can work on, but I am uncertain about it.
The other issue for complex life is multicellularity. Interestingly, recent experiments suggest that this is possibly easy to evolve, as it has been done in the lab. Therefore I would guess that complex life will evolve when possible. Could it evolve with anerobes? The only evidence we have is that there are no multicellular anerobes, and that anoxic conditions do not seem to allow evolution of such types. This suggests that if aerobes evolve, only they can become multicellular.
Alex T.
Yes only aerobes can become multicellular and in our neck of the
woods the ATP yield via O2 respiration provides energy in abundance,
This seems to be a high requirement of even modestly complex life.
So yes in a Earth analogue w/0 photosynthesis, expect microbial
mats, and I suspect prokaryotes will be the sole type of microbial life
in the vast majority of them.
@Eniac – Whilst plants produce oxygen, they cannot survive in anoxic conditions. You might consider why, with at least a billion years of photosynthesis, plants did not colonize the land until O2 levels were higher.
Plants respire aerobically, just as animals do, as the mitochondria are very similar. Remember too that the O2 output of cyanobacteria was trapped by iron until that sink was exhausted, and then free oxygen started to build up.
Now it might be possible that on a different world, that evolution allows organisms to produce and trap their own O2 for respiration, but it would require some structural differences because of the different solubilities of O2 and CO2 in water. In addition, photosynthesis would have to be dominant over the lifetime of the organism, which might make ecological expansion difficult (e.g. low seasonality).
One other thing to bear in mind. On Earth, Eukaryotes (us) , evolved by a chance incorporation of bacteria that became our energy producing mitochondria. Plants repeated the trick with organisms that became chloroplasts. The point is that plants did not separately evolve the trick of photosynthesis and aerobic respiration, but rather they had evolved separately, and were “captured” by cells that became the multicellular life on Earth. It may need a potentially different mechanism for hypothetical plants living on an anoxic world yet trapping the O2 almost completely for their own respiration. And that is apart from considering how the balance might be contained to prevent net O2 release and eventual atmospheric accumulation as we have on Earth.
I therefore agree with RobFlores that O2 is necessary for complex life, although I would maintain it is not a sufficient condition and that evolution has to find the trick of aerobic metabolism as well.
My thinking exactly.
There’s some evidence that another oxygen catastrophe, in the other direction, is possible and may have begun already.
Mathematical Modelling of Plankton–Oxygen Dynamics Under Climate Change
http://link.springer.com/article/10.1007%2Fs11538-015-0126-0
“We show that sustainable oxygen production is only possible in an intermediate range of the production rate. If, in the course of time, the oxygen production rate becomes too low or too high, the system’s dynamics changes abruptly, resulting in oxygen depletion and plankton extinction.”
New observational evidence suggests that the mass extinction of plankton may be underway:
Changing recruitment capacity in global fish stocks
http://www.pnas.org/content/early/2015/12/09/1504709112
“Across regions, we estimate that average recruitment capacity has declined at a rate approximately equal to 3% of the historical maximum per decade. … The extent of biological change in each large marine ecosystem is significantly related to observed changes in phytoplankton chlorophyll concentration and the intensity of historical overfishing in that ecosystem.”
@Eniac
“Oxygen is indeed not everything. There are large terrestrial lifeforms that produce rather than consume oxygen. They do for land what cyanobacteria do for the sea. They are called trees.”
Not to be picky but trees most definitely consume oxygen and would die as surely as you or I without a high O2 environment. Plants respire for the full 24hrs in a day, all day every day. They do however then photosynthesize during daylight to produce an excess of O2 (this was picked up on by Florence Nightingale who instructed all her nurses to remove all the plants/flowers from the wards during the night as they would use up some of the patients oxygen).
Where do stromatolytes fit into the article above? The O2-rich atmosphere before it fell to todays 21% was generated by anaerobic organisms that produced oxygen as a waste product… poor, unsuspecting fools that they were.
@Jim. Good references. Here is another showing phanerozoic data.
Note that O2 has been declining for about 100my and that the authors’ calculations show that Earth’s habitability has been declining too.
I too worry that phytoplankton productivity may be declining. Couple that with deforestation and farming and we could be in for a problem. But note how low O2 levels were after the Triassic extinction (~ 200 mya) which did not recover for a 100my.
One bright spot is that O2 remained high despite multiple recent glaciations, which suggests that photosynthesis remained in relative balance with respiration. Whether that stability continues in our Anthropocene is another matter.
I like Joy’s point that we tend to forget that even living planets won’t be very “touristy” as life will be likely microbial mats. We tend to depict Earth analogs as sometime in the last 450my, yet the probability is that such worlds will look like Earth before then. No lush forests, no terrestrial macro animals, no primitive ETIs. Just bare continents, and unicellular life where water is available. For much of that time, O2 levels will be very low and possibly hard to detect spectroscopically from Sol.
@Alex, thanks for the link, it’s a neat idea to create a “habitability” metric. This approach reminds me of another, rather terrifying, study from June:
“Earth is a chemical battery where, over evolutionary time with a trickle-charge of photosynthesis using solar energy, billions of tons of living biomass were stored in forests and other ecosystems and in vast reserves of fossil fuels. … With the rapid depletion of this chemical energy, the earth is shifting back toward the inhospitable equilibrium of outer space with fundamental ramifications for the biosphere and humanity.”
http://www.pnas.org/content/112/31/9511.abstract
I fully agree with Alex Tolley and RobFlores.
And I would like to add that there is some elegance in the idea of a geological clockwork mechanism: an earthlike terrestrial planet (of iron-silicon composition) has to be ‘ripe’ for higher life to appear, i.e. the great oxygen sinks, in particular the crust, first have to be saturated, which apparently takes some 3 – 3.5 gy.
This also reminds me of another very interesting article on CD a while ago about the ‘window of opportunity’ of various stellar spectral types with regard to (higher) life on a planet in the HZ. If I remember well, that post only pertained to the stellar continuous HZ, but this adds to it: the window of opportunity for higher life on a planet in the HZ is determined and constrained not only by the time period that the planet resides within that HZ, but also by the geological and geochemical time constraints of the planet itself.
In other words, a planet must be able to remain within the HZ long enough to allow for its O2 sinks to saturate and make higher life possible.
On earth it will probably become too hot for higher life in another 0.5 gy orso (about as long as we have had since the beginning of the Cambrian, making the entire window for such life on an earthlike planet around a G2 star about 1 gy. The brighter the star the shorter this window, because of rapid stellar evolution and the HZ moving outward. For stars of earlier spectral type than about F9/G0 the time window will probably always be too short for higher life.
Later G stars and K stars will do much better.
@Alex Tolley Low levels of oxygen are not hard to detect spectroscopically. It’s the technology that we don’t have yet. A one percent oxygen level or less is easily detectable with a large enough telescope or space telescope so it is not the amount of oxygen that makes it difficult to detect but only the amount of light we get of the exoplanet.
I also would not make the assumption that we don’t have convergent biological evolution given an Earth-Moon twin in the life belt of another solar system which also depends on the age of the star. There are many G type stars that are older than our Sun or near its age. Consequently, I imagine that there are many Earth twins out there in different stages of evolution. A planet with dinosaurs, one with human life in the stone age, and even at our level of advancement or more advanced.
It is our fossil fuel burning, deforestation and the resulting oxidation of nitrogen from our replacing them with pasture land, plantations, etc that is helping with the oxygen decline.
Oxygen was absolutely necessary for our plant to evolve since it evolved from algae and the oxygenated environment from them. Plants came from algae and the algae had to come first, the primitive extremophiles thrived on an environment with a lack of oxygen and of course the cyanobacteria.
Finally, ultra-violet radiation in the upper atmosphere converted oxygen into ozone which protects life from harmful ultra violet radiation.
The Oxygen had to come from plants. It had to be in the atmosphere first; The ozone had block out the UV before “land based life.” https://en.wikipedia.org/wiki/Timeline_of_the_evolutionary_history_of_life
@Geoffrey Hillend
There has been debate amongst some biologists over the randomness of evolution. It has been argued that rerunning evolution could turn up many of the same forms, but this is generally considered unlikely. Our own phyla, the chordates, could easily have disappeared, removing the whole line of major classes that we associate with evolutionary periods, like fish, amphibia, reptiles, etc.
I tend to align myself with those who see evolution as highly random. That rerunning evolution will result is very different outcomes. We may well see some phyla body plans re-emerge, and we certainly see convergent evolution in outward form, e.g. sharks, ichthyosaurs and whales. But I would suggest that an independent evolution of a humanoid biped is extremely unlikely, and a human mammal next to impossible. This re-running of evolution assumes a common starting point in molecular biology and biochemistry. If we had to re-run from the genesis of life, we may even get a very different evolutionary path that is unrecognizable to us.
If life turns out to be common, I would expect most worlds to be populated only by single cell organisms, a minority with macro-life, and possibly zero with intelligent aliens in our galaxy. Those ETIs will likely not look even remotely like us, and possibly more alien to us that any existing terrestrial life forms.
RobFlores:
How can you be so sure of this? This may be how it worked on Earth, but it is an entirely other thing to turn it into a general statement. Hard to show, because it is a negative assertion, and there is only one sample.
The example of multicellular plants show that energy alone is not a sufficient explanation. Plants generate more oxygen than they consume, which means they produce more energy through photosynthesis than through respiration, which proves that photosynthesis produces sufficient energy to operate multicellular organisms.
The reason plants have respiration is because this is the most convenient way to store energy overnight. It is not the only way, though. You could bind the oxygen you produce during the day, or you could use a less volatile oxidant for energy storage.
To me, the following is quite plausible: Assume that for some geochemical reason, such as more reducing conditions that you might find on a larger planet, free oxygen were to never evolve. Nevertheless, a Eukaryote-like combination evolves by the internalization of chloroplasts, forming complex plant cells that are incapable of respiration, but capable of going multicellular and colonize the land.
Name me a good reason why this could not happen, recalling that low power density is not one.
Geoffrey Hillend:
The only mention of ozone that I can find in this reference is this:
Interestingly, reference [27] mentions ozone only once and does not support the assertion that land life without it is not possible. Quite the opposite, actually: The authors argue that there was life on land long before oxygen or ozone.
Where else do you derive your certainty from regarding the statement “The ozone had [to] block out the UV before and based life.” ?
Alex Tolley:
Interesting question. As the article from the reference [27] cited above says, there was indeed life on land before oxygenation. Also, most photosynthesis today happens on land, so to some extent land plants helped the oxygenation rather than the other way around. Thirdly, oxygenation and the rise of multicellular organisms may have gone hand-in-hand, but that does not necessarily mean that one requires the other.
The proximity of the America’s is geographically incorrect for the described time periods.
Further to Joy’s and Alex’s statement that the vast majority (80, 90%?) of habitable terrestrial planets around solartype stars will probably be in a primitive state, i.e. primordial atmosphere with either no life (yet) and virtually no O2, high CO2, or only primitive bacterial life and at most low ( < = about 3%) O2, still high CO2.
This is not only be disappointing: such planets would be kind of ideal for human settling and terraforming, almost ready for the picking.
And it would also largely eliminate the moral issue concerning the settling of inhabited (meaning with more than just bacteria) planets.
I can imagine an advanced civilization only colonizing the uninhabited/bacterially inhabited planets and leaving those few that are already inhabited by higher life alone or at the most for scientific study.
Alex Tolley:
I agree with this, but I would add that there is nothing that says you need human mammals for intelligence. A rerun of evolution would certainly not produce human mammals, exactly. But it would likely produce organisms just as complex, filling similar niches, and having the same chance of developing culture and technology.
Possibly. Personally, I think they would be like us in a similar way as dolphins are like fish: Striking superficial similarities with large structural differences underneath. Of course, with alien life forms the differences would run much deeper, all the way down to the fundamental molecular biology. Nevertheless, I would expect the superficial, adaptive similarities to be of the same degree as between dolphin and fish.
Siegfried Franck has suggested that biospheres have a “life span”, and that the reign of multicellular life is limited:
“We ?nd that from the Archaean to the future a prokaryotic biosphere always exists. 2 Gyr ago eukaryotic life ?rst appears. The emergence of complex multicellular life is connected with an explosive increase in biomass and a strong decrease in Cambrian global surface temperature at about 0.54 Gyr ago. In the long-term future, the three types of biosphere will die out in reverse sequence of their appearance.”
Causes and timing of future biosphere extinction
http://www.biogeosciences-discuss.net/2/1665/2005/bgd-2-1665-2005.pdf
So it’s possible that if 90% of habitable planets are dominated by prokaryotes, many of them have passed through their eukaryotic and multicellular epochs already.
It seems to me that O2 levels, over the long term, is much more dependent on Geophysical aspects of the Earth, overlaying the impact of recent Geologically speaking ) human activity.
Yes, deforestation will have an impact. But what about volcanism. If the trend is downward as the Earth’s core cools, does not that imply that the recycling of nutrients slows. Organisms that produce O2 dependent on these nutrients would be impacted too.
@Eniac. You may remember this fanciful idea for an intelligent dinosaur that may have evolved if they had not been wiped out.
We should remenber that:
1. We now know birds are really dinosaurs that survived the KT event and that they never evolved high intelligence, although some like Corvids are pretty bright.
2. A rerun of evolution might not even have chordates again, so the intelligence of cephalopods might be as good as it gets. Unless squid somehow evolved to become terrestrial species, they could never become a highly technological species despite their intelligence and ability to manipulate objects. If you look at the major phyla, none have converged physical forms that look even remotely like us. (The movie Mimic has arthropods mimicking humans in a clever way as a clever SF idea).
OTOH, as I have mentioned before, the late John McCarthy thought that intelligence was convergent, although I think that was based on poor analogies and precious little evidence. If so, that would be good for SETI’s efforts.
The intelligence issue is probably the most important, as humans are on the cosmic threshold of being able to manipulate genomes like clay, as well as the singulatarian idea of mind uploading. Immaterial minds without local embodiment would be physically very different from evolved biological forms, but their minds might be able to communicate with us if they share enough references.
To turn the argument on its head, I have argued that machines intelligence is the most likely embodiment of mind to explore and colonize the galaxy. Their bodies could therefore resemble any other intelligence. If so, we might well find human looking intelligences discovering us first, and thus imitating the art of cheap SF movies. Or “we” might do the same for intelligences in the galaxy.
@Ronald. We may avoid planets with any sort of life to prevent contamination with our biology.
Terraforming takes a long time – thousands of years. The idea of “shake and bake” terraforming (Alien II) or even a “genesis effect” (ST: The Wrath of Khan) is not realistic. Unless a planet has physical parameters almost exactly like Earth, terraforming might not even work without whole ecosystems gene engineered to meet the requirements of the planet.
I tend to think that biological humans are better off building space habitats that conform to our biological needs, using the resources of another star system. We might visit the planets, but not alter them to become Earth like.
@Eniac. Life emerged onto land during the Silurian (410-440 mya). Oxygen levels had been climbing and were in the 15%+ range by then. Oxygen therefore preceded terrestrial life, indeed all multicellular life. Early multicellular life like sponges can tolerate relatively low levels of oxygen as they are sessile. Motile macrolife needed the energy of aerobic respiration and thus only evolved once O2 was high enough to provide a good concentration in shallow waters.
@Eniac – the requirement for energy is the driver for macrolife, whether sessile or motile, as RobFlores has pointed out.
While we can speculate about anaerobic macrolife, there is no fossil record evidence of such forms. The energy output of anaerobic respiration is just too low to support macro life as far as we know. I can imagine that soft, very thin anaerobic macrolife could form that would leave no fossil evidence, but if so, their lineage went extinct as there is no evidence in the tree of life for even a remnant of that biology beyond microbes. Microbial biofilms is as far as anaerobes got.
@Eniac
On Earth, cyanobacteria evolved to harness both photosynthesis and aerobic respiration. They may have used predominantly anaerobic respiration during the period when O2 levels on earth remained low due to the sinks, or they make have adapted to harness the cellular O2 directly for respiration. I can certainly imagine cyanobacteria oxygenating the local environment around the cells.
Thus I would expect such forms to remain no more than colonial and filamentous forms, unless there was adaptive value in creating larger structures. I could imagine them becoming more like fungal hyphae, spreading out and retaining the O2 in spaces within the hyphae. Perhaps they evolve similarly to terrestrial plants, except that gas exchange via stomata is done by other structures so that CO2 can be absorbed, but O2 kept resident for longer to aid respiration. If so, could animals evolve to take advantage of that trapped O2 for aerobic respiration? An interesting idea for world building for an SF story.
Alex Tolley. Evolution is not completely random. The randomness comes from lifes adaptability and mutations from the DNA. These are not completely random but have certain physical limitations due to physical reality which are based on survivability and necessity. That which makes not sense for the environment dies and does not pass on its genes. Consequently, life must follow convergent evolution everywhere in the universe since certain scientific universal principles and rules are based on the laws of physics and physical limitations which are the same in any environment and work everywhere such as quantum physics relativity, and there would be no way to predict anything that was completely random. .
Furthermore, the idea that UV light turns our oxygen into ozone is considered fact and no life can live on a planet without UV protecting ozone which is commonly accepted atmospheric chemistry and is widely supported.
Plants need oxygen and get it from the soil.
Alex Tolley December 23, 2015 at 15:53
@Ronald. ‘We may avoid planets with any sort of life to prevent contamination with our biology’.
That is basically what I said, although settling of planets with only bacterial life may be acceptable, or even unavoidable, if it is the vast majority of habitable planets. And not much chance of us eradicating all indigenous bacterial life.
‘Terraforming takes a long time – thousands of years’ (…)
I never said it would be very quick, but centuries or a few thousand years is very little for what you get: a whole planet.
‘terraforming might not even work without whole ecosystems gene engineered to meet the requirements of the planet.’
Required bio-engineering: near-term technology.
‘I tend to think that biological humans are better off building space habitats that conform to our biological needs’
No! Classical error: although space habitats will undoubtedly be used a lot and more and more, for scientific and industrial purposes, they are and will be very (VERY) expensive per unit area and life time in comparison with planets, and inherently quite instable, requiring continuous maintenance and replacement. Planets are very large, inherently long-term stable habitats. Furthermore, we humans like planets :-)
Jim Galasyn December 23, 2015 at 11:57
“Siegfried Franck has suggested that biospheres have a “life span”, and that the reign of multicellular life is limited (…)
In the long-term future, the three types of biosphere (prokaryotic, eukaryotic life, complex multicellular life) will die out in reverse sequence of their appearance.
So it’s possible that if 90% of habitable planets are dominated by prokaryotes, many of them have passed through their eukaryotic and multicellular epochs already.”
Yes! That is exactly what I meant in my comment of December 22, 2015 at 18:44
And the brighter the star (earlier spectral type) the shorter the 3 stages, down to the point where the complex multicellular stage disappears completely.
Alex,
On a world without free oxygen, all life would be anaerobic and oxygen a highly poisonous byproduct to be dissipated as quickly as possible. I do not think there would be much value in accumulating it. There are easier ways to store energy.
As you say, motile animal life is unlikely without respiration, but I see no reason why there could not be complex anaerobic plants. There are always niches where there is adaptive value in creating larger structures, especially on land. Think about cacti, which are almost like spaceships manned by the cells within.
Maybe, in the absence of animals to compete with them, motile plants might even evolve. Slow ones, probably, to stay within the energetic limitations of photosynthesis.
Alex:
I am talking about photosynthesis, not anaerobic respiration. We know its output is sufficient for macro life, as complex plants derive all their energy from it. Any respiration they may do has to be reversed, later, so provides no net energy. And photosynthesis can of course be anaerobic, it would have had to be before oxygenation.
@Eniac – your wording is a little ambiguous. Photosynthesis doesn’t create any energy for plants – it just fixes carbon to sugar. The sunlight is the energy needed to do this. Respiration of the stored sugar is then used to drive all the the biochemistry to all the plant to grow and eventually reproduce. All eukaryotic plants from single celled algae to redwoods are aerobes.
If anaerobic respiration was sufficient to create macro plants, we might expect there to be some evidence of it, e.g. in the genomes of photosynthetic bacteria. While absence of evidence is not evidence of absence, I would have expected there to be some clues in living organisms that this line of evolution had occurred. Again, there was more time to evolve macrolife anaerobes that there has been since the beginning of the Cambrian when there was a flowering of phyla. One might expect that if macro anaerobes were possible, they would have evolved in all that time without aerobic competitors.
So while I cannot rule out macro anaerobes on one of the exoplanets, the lack of evidence on Earth suggests that RobFlores’ argument is essentially correct, IMO.
Now that we have such exquisite gene editing technologies like CRISPR, it may just be possible to re-engineer some early macro plants like kelp or mosses to see if they could operate on anaerobic respiration.
Alex:
All the energy to drive plant growth comes from the photosynthetic reaction center, in the form of an H+ gradient that is then converted to ATP by ATPase. As far as I can tell, this ATP is directly available to drive all the plant’s biochemistry, as long as there is light.
Before any respiration can happen, the plant must synthesize glucose, which takes more energy than can later be recovered by respiration. Respiration therefore can at best be an energy storage mechanism, to get over periods with no light. It does not contribute any extra energy whatsoever, because plants do not have the luxury of burning biomass that animals have. They must create it, instead, in order to grow.
From this it is very clear that the energy generated by the photosynthetic reaction center coupled to ATPase is sufficient to maintain complex plants. You are right, it does seem that respiration is linked to multicellularity by association. If there is a good reason for that, though, it must be something other than the amount of energy available. I have not seen such a reason offered, so I must assume that complex plants with photosynthesis and without respiration are possible, at least in principle. Quite possibly, the association of multicellularity and respiration in plants is coincidental and maintained only because respiration happens to be a good way to store energy under oxygenic conditions.
Maybe I am missing something, please let me know what it is.
Maybe macrolife happened after oxygenation just because it took a long time to evolve, or maybe its evolution was triggered somehow (rather than enabled) by oxygenation. Once there was free oxygen in the water and air, there would have been very few niches for macrolife without oxygen, and there would have been little reason to evolve an energy storage mechanism that would work without oxygen.
Alex: Here is a great article about photosynthesis and respiration in cyanobacteria: http://www.synechocystis.asu.edu/pdf/photosyn_%20resp.pdf
Among many other things, it says this:
In other words, photosynthesis is the primary and more powerful source of energy to the cell, respiration is a secondary backup used to maintain the organisms activity during dark periods.
If, as you say, “the requirement for energy is the driver for macrolife, whether sessile or motile”, the photosynthetic electron transport chain can clearly be this driver, respiration (and therefore oxygen) are not required.
You are likely correct if you modify your statement to be about “animal macrolife”, i.e. excluding plants. When there is fuel to burn, respiration is by far the most powerful source of energy in biology, enabling the energy intensive motility of animals. But plants are macrolife, too, and their energy requirements can obviously be met by photosynthesis, alone. The fact that there aren’t any plants that do not respire, if indeed true, requires a different explanation.
@Eniac
two articles on anaerobic photosynthesis
1. Anoxygenic Photosynthesis
2. Anaerobic Photosynthesis
No one is disputing that photosynthesis generates energy. Whay is disputed, based on evidence, is that multicellular plants have not evolved under anoxic conditions. I would also contend that there was enough time for them to do so before aerobic plants evolved.
This doesn;t mean that a priori they cannot evolve, but rather their absence suggests that evolution of such forms beyond colonial types was not favored. This may be due to the downsream biochemical reactions being insufficient to support such macro life, but I admit that this is speculation on my part.
As far as exoplanet life is concerned, I am always wary of assuming terrestrial life is the “gold standard” model to base all assumptions on. I personally would be thrilled if we discovered fundamentally different life to study. I suspect however, that synthetic life experiments may get there first, given the rate of progress in this area. These life forms may even be designed by the new, more powerful, AI systems coming on stream. I could easily see a “Watson” coming up with possible ideas to design experiments around. The sheer complexity is ideally suited to such machine intelligences.
Alex: Thanks for those articles. Both are about photosynthesis that does not produce oxygen. We are speculating about photosynthetic macrolife that does produce oxygen, but does not require it. You have convinced me that such plants do not exist, but I continue to believe that they could and probably would exist had free oxygen never accumulated on Earth.
There would have to be an alternative mechanism to store energy for dark periods. Perhaps a less volatile oxidant, such as hydrogen peroxide, or a solid peroxide. Or, to circumvent respiration entirely, an electrochemical reaction involving metal ions such as Zinc or Lithium, as in batteries. On tidally locked planets, you may also have permanently lighted environments, where dark-adaptation is not needed at all.
Isn’t this exceedingly obvious? Oxygen is produced by chlorophyll precursors, but they need minerals so they live in the shallow water zone where they can extract minerals while still getting photons from above. The waters circulate and oxygen diffuses through water, so the time necessary to get shallow water creatures to fill up the oceans with dissolved oxygen has to be huge.