We as humans have three fundamental questions. Where do we come from? Where are we going? Are we alone in the universe? The answers to these thrust at the core of our humanity and uniqueness. Through science we seek out replies to these inquiries.
The Drake Equation
In 1960 the National Academy of Sciences asked Frank Drake to gather a group of scientists to discuss the search for extraterrestrial intelligence, the program we now call SETI.
As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.
— Frank Drake (ref)
What emerged was the Drake Equation
where:
N = the number of civilizations in our galaxy with which communication might be possible;
and
R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
It is ne = the average number of planets that can support life per star and fℓ = the fraction planets with favorable conditions to develop life, that I have been the most intrigued by. ne is related to the habitable zone, the region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface. More specific criteria can also be used which can also alter the percentage from 0.5-20%. Some estimate fℓ at 100%; where life can evolve it will. In our sample size of one, Earth, life arose.
Conversely, some argue that the value of fℓ approaches 0%. Life as we know arose only once on earth; all life has a single origin. This points to a set of specific and very rare conditions unseen at other points in space and time in Earth’s history. The chances of this set of conditions occurring on other planets are equally rare. This concept is formalized as the Rare Earth hypothesis; the emergence of complex multicellular life on Earth required a combination of events and circumstances so rare as to only occur once.
The Deep Sea
On the brink of Cameron being only the third person to ever visit the Challenger Deep in the Marianas Trench, I’m reminded how much exploration and scientific inquiry of the deep sea continues to challenge our perceptions of life and how life works. Insight gained form observing and investigating deep-sea life forced us to redefine and reexamine our theories of life and potentially push fℓ and ne higher. Life, and the conditions for life, may not be as rare as we think.
Life Does Not Require the Sun
Geologists exploring the seafloor near the Galapagos Rift in 1977 discovered a biological system wholly different than anything encountered on earth before. Later that year Peter Lonsdale published the first paper on the unique life at hydrothermal vents.
A community of abundant suspension-feeding organisms was photographed around an active hydrothermal vent at the Galapagos Rift. A site on the crest of the East Pacific Rise where hydrothermal discharge is suspected also has a dense colony of sessile organisms. The high standing crop of macrobenthos in these patches probably results from local increases of deep-sea food supply near hydrothermal plumes in the bottom water.
The venting hot water is rich in variety of minerals, heavy metals, and toxic substances including hydrogen sulfide. Some bacteria have the ability to use the oxidation of hydrogen sulfide to provide energy through a mechanism other than photosynthesis. These bacteria, whether free living or in symbiosis with such organisms as clams and tubeworms, form the base of food chains.
Life Can Survive On Minimal Food
The lack of light in the deep prohibits photosynthesis and animals, not at vents, rely upon the minor amount of food that may sink from the productive ocean surface.
Each year approximately 16 gigatons of carbon fixed by phytoplankton sink to the ocean interior. This amount is a mere 3 percent of the total produced at the ocean surface. Consider that 3 percent of a five-pound bag of sugar is less that five-and-a-half tablespoons. This small amount of fluxed carbon, carried largely by “marine snow,” dusts the seafloor and represents the only food source for the majority of organisms in the deep
Yet life thrives in the deep sea and perhaps in the ultimate conundrum the deep sea floor abounds with profound biodiversity. Adaptations flourish to allow animals to find rare mates, utilize novel food sources, reduce energy spent looking for food, and reducing overall energetic costs.
Life Can Survive at Environmental Extremes of Temperature, Pressure, and Toxicity
The mere presence of animals at near freezing or near boiling at hydrothermal vents, up to 1100 atmospheres of pressure, and at vent levels of hydrogen sulfide and heavy metals that would kill most animals, is more than sufficient data to suggest that life can survive in even the most hostile of conditions. But how?
Deep-sea organisms demonstrate quite well that extremes of temperature, pressure, and toxicity can be adapted to. Pressure increases 1atm for every 10m, so deep-sea organism can experience a range of pressures from 20atm at the shallowest point to 1100atm in the deepest. This results in a tighter packing of the phospholipids of cellular membranes which lower the permeability of the membrane. But wait it gets worse…Temperatures in the deep are typically near 4 degrees C and near the poles water can become supercooled to -1 degree C. This also decreases the permeability of the cell membrane. Deep-sea organisms deal with this by increasing the percentage of unsaturated fatty acids. Unsaturated fatty acids have kinks in their tails, due to double covalent bonding of carbons, that prevents tight packing in the membrane.
We also see that temperatures and pressures select for enzymes with different temperature sensitivities and pressure resistances. Changes in protein structure can influence their cellular function. Thus selection in deep-sea animals has been for rigidity to counteract pressure. Proteins contain hydrogen and disulfide bonds between different subunits and parts of the amino acid chain that both dictate structure. A selection for proteins with increased bonding would minimize changes in shape to do pressure.
Extreme temperature gradients experienced at hydrothermal vents also present unique challenges. For example, hydrothermal vent worms such as Alvinella can experience 22 degrees C near the gills and 60+ degrees C near the body trunk.
Metazoans Are Possible Without Oxygen
Unique environments with in the deep sea such as anoxic/hypoxic regions (i.e. zero or little oxygen) require at a minimum alterations to proteins to increase oxygen binding efficiency and transport. But does complex multicellular require oxygen even if it is minimal. Research that I was involved with suggests that increases in oxygen were prerequisite for larger sized multicellular organisms.
Several unicellular organisms (prokaryotes and protozoa) can live under permanently anoxic conditions. Although a few metazoans can survive temporarily in the absence of oxygen, it is believed that multi-cellular organisms cannot spend their entire life cycle without free oxygen.
Yet in research last year, three species of Loriciferans (multicellular organisms with heads, mouths, digestive systems, complex life cycles, and separate sexes) were found in completely oxygenless areas of the Mediterranean Sea. These species lack oxygen-requiring mitochondria and instead have organelles called hydrogenosomes, similar those of anaerobic bacteria. As the authors note,
This is the first evidence of a metazoan life cycle that is spent entirely in permanently anoxic sediments. Our findings allow us also to conclude that these metazoans live under anoxic conditions through an obligate anaerobic metabolism that is similar to that demonstrated so far only for unicellular eukaryotes. The discovery of these life forms opens new perspectives for the study of metazoan life in habitats lacking molecular oxygen.
Photosynthesis Does Not Require Light From the Sun
We all learned that photosynthesis requires sunlight back in grade school. Thus life would be confined to habitats with solar light available. Yet work in 2005 on green sulfur bacteria isolated from hydrothermal vent, an organism that requires light for growth by the oxidation of sulfur compounds to reduce CO2 to organic carbon. The source of light at a hydrothermal vent for green sulfur bacteria running on photosynthesis? Geothermal radiation that includes wavelengths absorbed by photosynthetic pigments.
Deep-Sea Origins of Life
Hypotheses for centers of origins of life on earth include primordial beaches, tide pools, hot springs, frozen ocean, atmosphere, and others. Of course there are competing models. Stanley Miller created an early earth analog in the laboratory that produced organic molecules from water, methane, ammonia, hydrogen, and shot of electricity. Miller and Urey’s experiments in 1952, although quite distant from demonstrating how life evolved, pointed to the possibility that the conditions on the young earth’s surface could produce the basic building blocks of life. They operate under the Oparin-Haldane hypothesis or the organic soup (or primordial ooze) hypothesis. The problem is that the early atmosphere would have to be reducing for these reactions to work. Recent evidence suggests this may not be the case. As well, some argue that CO2 and/or CO would reduce organic material by chemosynthesis. So if the earth as a whole was not reducing, then you would need particular environments to be.
Fast forward to the 1970′s and the discovery of hydrothermal vents, a reducing environment, which spawns a new line of thinking. The interface of cold and hot waters allow for unique reactions to occur. Moreover, the extreme pressure, protection from UV radiation, abundant geothermal energy, and both methane and sulfide provide the necessary conditions to serve as a cradle of life in the deep depths of the ocean. Of course attacks on this theory suggest that life could have begun at ocean vents, saying high temperatures would have destroyed amino acids. Van Dover in her book on vents, points to both empirical and laboratory evidence indicating this is not the case. Van Dover also presents an excellent figure of phylogentic tree of Bacteria, Eucarya, and Archaea, that point to the hyperthermophilic nature of the basal taxa. Add to this a study from Geology demonstrating that types of clay mineral can convert simple carbon molecules to complex ones in conditions similar to the hot and wet environment of hydrothermal vents. The group simulated a vent in the laboratory by immersing various types of clay in pressurized water at 300 °C for several weeks and looking at the fate of methanol, a compound formed readily formed at vents. Having helped such delicate molecules to form, the clays can also protect them from getting broken down in the piping hot water issuing from the vents.
Evolution is Clever
One has to be in awe of what life through evolution has accomplished on our planet. The examples above are a mere fraction of the ways deep-sea life has adapted to extremes. In the 1800’s, the belief was that the deep was inhospitable to life. In the early 1900’s, the belief was deep-sea life was present but not diverse or abundant. Fast forward to today, we have a much different view of the deep oceans. A view that expands our thinking on what life is capable of and suggests that the planetary conditions needed to support life and the potential for life to develop are much greater than we have thought previously.
So, a change in ne and fe, eh? By how much? This totally has a short PLoS One thought piece written all over it.
(although I think our knowledge of the intelligence of things like squid should totally cause us to rethink fi as well…devious lil’ bastards)
Once we actually get out an explore, I would be shocked if we didn’t discover that life not only exists elsewhere in our solar system (let alone the universe), but is fairly cosmopolitan.
Great post!
However, I do have a problem with this assumption:
“Life as we know arose only once on earth; all life has a single origin. This points to a set of specific and very rare conditions unseen at other points in space and time in Earth’s history.”
The fact that current biological surveys all point to one phylogenetic origin of life does not necessarily mean that life arose only once – it may just as easily mean, if we take evolutionary principles into account, that life arose many times, but that its current form was the most successful one, and out-competed the rest of the lifeforms that arose over the earth’s history – with some sort of significant advantage over the others.
Naturally, without hard scientific facts to back up this idea it remains an unfounded theory, but it is an interesting thought experiment.
Sweet post! I had NO idea there were photosynthetic bacteria down there–it’s crazy but it makes perfect sense! In general, this reminds me of a recent SciAm article suggesting that in a few billion years, the universe may be more life-friendly than it is now. As one of my friends said, “I’ll put it on my calendar.”
Under “life can survive on minimal food”, I think when you wrote “the deep sea floor is abound with” you meant “the deep sea floor abounds with.” And when you mention loriciferans, it would help to have dashes or parentheses around the descriptive phrase “multicellular organisms with . . . separate sexes”.
Thanks for this! I too had somehow missed the anaerobic photosynthesis in vent bacteria, though I knew bacteria may have such systems capable of capturing well into the IR.
This article seems to cover all the bases, including not confusing non-solar power sources with non-oxygenic. In many cases the dependence of photosynthesis is still there, albeit indirectly.
And permit a student of astrobiology to note that there is a difference between rare life and rareness of complex multicellulars, and the short period of time before life developed here implies it is a frequent and/or easy attempt.
I’ll add a note of caution on phylogenetic estimations of thermophily. It seems there is heavy critique launched against anything short of whole genome methods such as used on protein families. Here is an example of early critique.
Those methods pick up a mesophilic root which later evolved into early thermophily in all three domains. Here is an example of work (though not the latest IIRC) deriving a non-hyperthermophilic LUCA. (It is thus exciting to contemplate a bottleneck, say life arising before the Late Heavy Bombardment. But I don’t think any evidence for a bottleneck is present as of yet.)