60 years of silence - so far

[lpetrich]

(on fungi that can get energy from ionizing radiation)

In the wild, that's a VERY weak energy source, so it's no surprising that it took nuclear reactors and the like to create environments where such organisms could thrive.

Why not ET with a taste for humans?

Is that at all likely? Why eat us instead of any other species on this planet?

For all we know, ET's might like eating octopus or jellyfish or moss or seaweed or mushrooms.

 

[steve_bank]

It is certainly possible, IMO. Just look at our ecosystem.

Generally divided into predator and prey.

One of my favorite Twilight Zone stories was To Serve Mam. It should be online. An ET arrives and improves agrculture and ends disease. In the end ET was turning the Earth into a cattle ranch.

 

[lpetrich]

"To Serve Man"

That begs the question. Why us and not seaweed?

Also, as food-animal species go, we are very inefficient. Pigs are much more efficient. I find various numbers on pig reproduction and growth, so here are some approximate values. A pig is typically pregnant with several piglets at a time, usually around 8, and a pig can have two pregnancies per year, each pregnancy usually lasting around 4 months. That means some 16 piglets per year. A piglet takes about 6 months to grow to a marketable size, around 280 lb / 130 kg, a little over typical human body weight.

Cows are not quite as efficient, and more like us, with typically being pregnant with one calf at a time, with one pregnancy per year, each pregnancy typically lasting 9 months. But a calf takes about 2 years to become full grown, with a weight of around 1,400 lb / 600 kg.

Chickens? A meat hen can lay nearly one egg per day, though often one egg every two days. A meat chicken takes 2 months to grow to maturity, and typically weights 4 lb / 2 kg.

 

[lpetrich]

Turning to the  Cambrian explosion one finds the first clear evidence of ancestors of many present-day animal phyla, though there are some oddballs that are difficult to place, like the archaeocyathids, some roughly conical sponge-like reef-building animals that went extinct in the middle of the Cambrian.

As to why it happened, my favorite hypothesis is the emergence of predators, animals that try to catch and eat other animals. That stimulated the evoluition of protective features like hardened skin and shells and burrowing -- and we find multiple evolution of such features.

Bivalve | Ohio Department of Natural Resources
Brachiopods vs Bivalves - Digital Atlas of Ancient Life

Consider bivalves and brachiopods.

They both have paired shells, and they look much alike if you don't look at their shells very closely and don't look at the shells' owners at all, but bivalve ones are originally left and right shells and brachiopod ones are originally dorsal and ventral ones. So the ancestors of bivalves and brachiopods evolved their paired shells separately.

There are some possible bivalves in the Cambrian Period, but bivalves became easily recognizable in the Early Ordovician period, a little later.  Bivalvia

Recognizable brachiopods go back to the Early Cambrian.  Brachiopod

Brachiopods were very common in the Paleozoic Era, more than bivalves were back then, but the Permo-Triassic mass extinction hit brachiopods much harder than bivalves, and that is why we eat clam chowder and not lampshell chowder.

 

[lpetrich]

It's  Vendobionta that's in the Ediacaran.

 

[SLD]

Turning to the  Cambrian explosion one finds the first clear evidence of ancestors of many present-day animal phyla, though there are some oddballs that are difficult to place, like the archaeocyathids, some roughly conical sponge-like reef-building animals that went extinct in the middle of the Cambrian.

As to why it happened, my favorite hypothesis is the emergence of predators, animals that try to catch and eat other animals. That stimulated the evoluition of protective features like hardened skin and shells and burrowing -- and we find multiple evolution of such features.

Bivalve | Ohio Department of Natural Resources
Brachiopods vs Bivalves - Digital Atlas of Ancient Life

Consider bivalves and brachiopods.

They both have paired shells, and they look much alike if you don't look at their shells very closely and don't look at the shells' owners at all, but bivalve ones are originally left and right shells and brachiopod ones are originally dorsal and ventral ones. So the ancestors of bivalves and brachiopods evolved their paired shells separately.

There are some possible bivalves in the Cambrian Period, but bivalves became easily recognizable in the Early Ordovician period, a little later.  Bivalvia

Recognizable brachiopods go back to the Early Cambrian.  Brachiopod

Brachiopods were very common in the Paleozoic Era, more than bivalves were back then, but the Permo-Triassic mass extinction hit brachiopods much harder than bivalves, and that is why we eat clam chowder and not lampshell chowder.

But predation was due to the evolution of the eye. I always viewed it as the eye that caused the Cambrian explosion.

Regardless it still took over 4 billion years to get to that point. Over 3 billion of life - just bacterial. That’s why I think advanced life will be very rare in our galaxy - maybe ten or so at any one time.

 

[lpetrich]

Let's see about our planet. Time from formation of our planet, some 4.5 Gy (billion years) ago

 Oldest dated rocks

• Oldest known mineral grain: 0.1 Gyr - a zircon grain from the Jack Hills, Australia
• Oldest known rock formation: 0.5 Gyr
• Oldest known cratons: 1.0 Gyr - a craton is a very old block of continental crust

Evolution of cyanobacteria:

• Advanced two- and three-dimensional insights into Earth’s oldest stromatolites (ca. 3.5 Ga): Prospects for the search for life on Mars
• Cyanobacteria evolution: Insight from the fossil record - ScienceDirect
• Frontiers | An Expanded Ribosomal Phylogeny of Cyanobacteria Supports a Deep Placement of Plastids
• Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils - Schirrmeister - 2015 - Palaeontology - Wiley Online Library
• Evolution of multicellularity coincided with increased diversification of cyanobacteria and the Great Oxidation Event | PNAS
• Frontiers | Evolutionary Patterns of Thylakoid Architecture in Cyanobacteria

Continuing on from formation:

• Oldest known stromatolite: 1.0 Gy
• Emergence of cyanobacteria: around 1.5 Gy
• Thylakoids, multicellularity, heterocysts: around 2.0 Gy
• The Great Oxidation Event: 2.0 - 2.5 Gy

An awfully long time for cyanobacteria to evolve, given how fast prokaryotes can multiply. I checked on how fast cyanobacteria can multiply: Cyanobacteria Poisoning (Blue-green Algae) | NDSU Agriculture - "Under favorable conditions, bacterial numbers multiply rapidly, doubling in one day or less"

A billion years is about 100 billion generations.

 

[lpetrich]

For eukaryotes, the oldest fossils that are likely of them are microfossils called "acriterchs" because it's hard to tell what they were fossils of.

Palaeos Proterozoic: The Proterozoic Era - has a diagram of acritarch evolution about 2/3 down. Acritarchs were approximately spherical, and they grew a variety of spines and similar projections from their surfaces, diversifying over time. They first show up around 1.7 billion years ago, and continue into the Cambrian, at least, some 500 million years ago. Thus, 2.8 to 4.0 Gy since our planet's formation.

The first clear fossil of a eukaryote is a red alga that was named Bangiomorpha pubescens It lived some 1.2 billion years ago, 3.3 Gy since the Earth's formation. Not long after that was Proterocladus antiquus a green alga that lived about a billion years ago, 3.5 Gy since the Earth's formation.

Relative to that, evolution was faster from the Ediacaran to the present, some 600 million years, but not much faster.

 

[lpetrich]

But predation was due to the evolution of the eye. I always viewed it as the eye that caused the Cambrian explosion.

Seems to me like it's reversing cause and effect. Being a predator stimulated the evolution of advanced eyes, so one could find one's prey more easily, and predation also stimulated the evolution of advanced eyes in some prey species, so they could see a predator coming.

Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes | Nature - "Their preserved visual surfaces are composed of at least 16,000 hexagonally packed ommatidial lenses (in a single eye), rivalling the most acute compound eyes in modern arthropods."

Anomalocaris canadensis grew to about 50 cm long, about 1.5 feet or 1 cubit. That's roughly the length of a trout or a lobster. But it was the biggest animal of its time -- and a predator.

High-resolution eyes evolved several times, and lens-camera eyes separately in vertebrates and in cephalopods.

 

[SLD]

But predation was due to the evolution of the eye. I always viewed it as the eye that caused the Cambrian explosion.

Seems to me like it's reversing cause and effect. Being a predator stimulated the evolution of advanced eyes, so one could find one's prey more easily, and predation also stimulated the evolution of advanced eyes in some prey species, so they could see a predator coming.

Acute vision in the giant Cambrian predator Anomalocaris and the origin of compound eyes | Nature - "Their preserved visual surfaces are composed of at least 16,000 hexagonally packed ommatidial lenses (in a single eye), rivalling the most acute compound eyes in modern arthropods."

Anomalocaris canadensis grew to about 50 cm long, about 1.5 feet or 1 cubit. That's roughly the length of a trout or a lobster. But it was the biggest animal of its time -- and a predator.

High-resolution eyes evolved several times, and lens-camera eyes separately in vertebrates and in cephalopods.

But how could they find prey if they could not see?

Guess it was a form of co evolution.

And in turn all of this created the need for hardened bodies, shells. And this is the real source of the Cambrian explosion.

 

[steve_bank]

Sounds like a which came first the chicken or he egg problem.

 

[bilby]

Sounds like a which came first the chicken or he egg problem.

That particular "problem" has always had a very obvious and objectively correct answer. Eggs pre-date chickens by at least 270 million years, even if we take the broadest possible definition of "chicken", and narrow the definition of "egg" to include only eggs with a hard calcium carbonate based shell.

 

[Swammerdami]

Sounds like a which came first the chicken or he egg problem.

That particular "problem" has always had a very obvious and objectively correct answer. Eggs pre-date chickens by at least 270 million years, even if we take the broadest possible definition of "chicken", and narrow the definition of "egg" to include only eggs with a hard calcium carbonate based shell.

Whether your science is correct or not, you are rejecting Thai culture. For Thais the question Which comes first, chicken or egg? has a very obvious answer.

To understand this solution you must know a little about the Thai alphabet. There are 44 consonants, with much duplication. When spelling a word aloud it may not be enough to specify the consonant, e.g. "khor." There are several different "khor"s. The 44 consonants are memorized in Kindergarten with the help of this song:



The song begins "Kor Oei Kor Kai. Khor Khai Yuu Nai Lao." Whether you understand Thai or not, you can plainly see at the 0:15 mark that "K as in Chicken" is the very first letter of the alphabet, and "Kh as in Egg" is the second. In fact you can see this in the still frame provided by the Youtube preview. Clearly the Chicken comes before the Egg.

Hope this helps.

 

[SLD]

Sounds like a which came first the chicken or he egg problem.

No. It’s not really that. It‘s an issue of coevolution. Prior to eyes, organisms still needed to eat. They may have filter fed, like corals do. They may have simply gotten lucky, but then developed the ability to detect light and dark. That helped catch things better, but prey would do better with that too. Suddenly it’s an evolutionary arms race to see better. Hard shells then develop to protect. And suddenly, these shells fossilize a lot better. Hence the Cambrian “explosion.”

 

[bilby]

Sounds like a which came first the chicken or he egg problem.

No. It’s not really that. It‘s an issue of coevolution. Prior to eyes, organisms still needed to eat. They may have filter fed, like corals do. They may have simply gotten lucky, but then developed the ability to detect light and dark. That helped catch things better, but prey would do better with that too. Suddenly it’s an evolutionary arms race to see better. Hard shells then develop to protect. And suddenly, these shells fossilize a lot better. Hence the Cambrian “explosion.”

The development of hard shells also depended on oceanic pH. In an acidic ocean, a calcium carbonate shell simply dissolves as fast as the animal can excrete it.

It doesn't matter how much protection you need from predators, if you're prevented by chemistry from building a shell, you won't evolve one.

 

[SLD]

Sounds like a which came first the chicken or he egg problem.

No. It’s not really that. It‘s an issue of coevolution. Prior to eyes, organisms still needed to eat. They may have filter fed, like corals do. They may have simply gotten lucky, but then developed the ability to detect light and dark. That helped catch things better, but prey would do better with that too. Suddenly it’s an evolutionary arms race to see better. Hard shells then develop to protect. And suddenly, these shells fossilize a lot better. Hence the Cambrian “explosion.”

The development of hard shells also depended on oceanic pH. In an acidic ocean, a calcium carbonate shell simply dissolves as fast as the animal can excrete it.

It doesn't matter how much protection you need from predators, if you're prevented by chemistry from building a shell, you won't evolve one.

Ok. But what do we know about the acidity of the Ediacaran oceans?

 

[lpetrich]

Regardless it still took over 4 billion years to get to that point. Over 3 billion of life - just bacterial. That’s why I think advanced life will be very rare in our galaxy - maybe ten or so at any one time.

There is a further problem. We emerged just in time, not long before it will likely become uninhabitable by us, and eventually by *any* organism.  Future of Earth

The Earth's average surface temperature has been relatively constant over geological time, and that is due to the  Carbonate–silicate cycle

• If the Earth's atmosphere has only a little bit of CO2, then it stays in the air and volcanoes add to it.
• If the Earth's atmosphere has a lot of CO2, then the resulting greenhouse effect warms our planet, causing more rock weathering, something that takes CO2 out of the air.

 Carbon dioxide in Earth's atmosphere with  File:Phanerozoic Carbon Dioxide.png
The preindustrial concentration of CO2 in our planet's atmosphere was around 260 - 280 ppm, and it was much greater in the past, though it's hard to find very precise numbers. It was nearly 2000 ppm over most of the Mesozoic, 1000 ppm in the Late Paleozoic, and 4000 ppm in the Early Paleozoic -- and likely even greater in the Proterozoic, Archean, and Hadean.

So this thermostat effect has been driving down CO2 concentration as the Sun becomes brighter. It was at 75% of present luminosity some 4 billion years ago, and it will continue to get brighter.

 

[lpetrich]

What will that lowered CO2 do? It will make it harder for plants to photosynthesize. Most land plants use  C3 carbon fixation to collect CO2, their ancestral process, shared with algae and cyanobacteria.

But some plants now use a different process for collecting CO2:  C4 carbon fixation This process has an advantage over C3 in conditions like drought, high temperatures, and N2 or CO2 shortage.  List of C4 plants has

C4 photosynthesis probably first evolved 30–35 million years ago in the Oligocene, and further origins occurred since, most of them in the last 15 million years. C4 plants are mainly found in tropical and warm-temperate regions, predominantly in open grasslands where they are often dominant. While most are graminoids, other growth forms such as forbs, vines, shrubs, and even some trees and aquatic plants are also known among C4 plants.[1]

Graminoid = grasslike plant, forb = herbaceous (non-woody), non-grasslike plant

C4 plants are usually identified by their higher 13C/12C isotopic ratio compared to C3 plants or their typical leaf anatomy.[5] The distribution of C4 lineages among plants has been determined through phylogenetics and was considered well known as of 2016. Monocots – mainly grasses (Poaceae) and sedges (Cyperaceae) – account for around 80% of C4 species, but they are also found in the eudicots.[1]

The article then listed taxonomic families of plants, and they are scattered over the monocots and eudicots. So it is evident that C4 evolution is a case of  Parallel evolution like scorpions and lobsters/crabs both evolving pincers. They have the same limb architecture, and pincers emerged in the same way in both groups.

"Within the next 600 million years from the present, the concentration of carbon dioxide will fall below the critical threshold needed to sustain C3 photosynthesis: about 50 parts per million."

But given how many times C4 photosynthesis has evolved, it is likely to evolve many more times, so the flora will not be as limited as the article suggests.

However, C4 carbon fixation can continue at much lower concentrations, down to above 10 parts per million. Thus plants using C4 photosynthesis may be able to survive for at least 0.8 billion years and possibly as long as 1.2 billion years from now, after which rising temperatures will make the biosphere unsustainable.[80][81][82]

...
The loss of higher plant life will result in the eventual loss of oxygen as well as ozone due to the respiration of animals, chemical reactions in the atmosphere, and volcanic eruptions. Modelling of the decline in oxygenation predicts that it may drop to 1% of the current atmospheric levels by one billion years from now.[87] This decline will result in less attenuation of DNA-damaging UV,[79]

...
Based on oxygen’s half-life in the atmosphere, animal life would last at most 100 million years after the loss of higher plants.[12] Some cyanobacteria and phytoplankton could outlive plants due to their tolerance for carbon dioxide levels as low as 1 ppm, and may survive for around the same time as animals before carbon dioxide becomes too depleted to support any form of photosynthesis.[12]

So we have about a billion years more before our planet becomes uninhabitable for us from lack of oxygen.

 

[lpetrich]

One billion years from now, about 27% of the modern ocean will have been subducted into the mantle. If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the current surface reservoir would remain at the surface.[56] Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C; 116 °F). The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans.[89][90] At this point, models of the Earth's future environment demonstrate that the stratosphere would contain increasing levels of water. These water molecules will be broken down through photodissociation by solar UV, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's seawater by about 1.1 billion years from the present.[91][92]

There will be two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere while water vapor starts to accumulate in the stratosphere (if the oceans evaporate very quickly), and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere (if the oceans evaporate too slowly). In this ocean-free era, there will continue to be surface reservoirs as water is steadily released from the deep crust and mantle,[56] where it is estimated that there is an amount of water equivalent to several times that currently present in the Earth's oceans.[93] Some water may be retained at the poles and there may be occasional rainstorms, but for the most part, the planet would be a desert with large dunefields covering its equator, and a few salt flats on what was once the ocean floor, similar to the ones in the Atacama Desert in Chile.[14]

With no water to serve as a lubricant, plate tectonics would very likely stop and the most visible signs of geological activity would be shield volcanoes located above mantle hotspots.[90][79] In these arid conditions the planet may retain some microbial and possibly even multicellular life.[90] Most of these microbes will be halophiles and life could find refuge in the atmosphere as has been proposed to have happened on Venus.[79] However, the increasingly extreme conditions will likely lead to the extinction of the prokaryotes between 1.6 billion years[88] and 2.8 billion years from now, with the last of them living in residual ponds of water at high latitudes and heights or in caverns with trapped ice. However, underground life could last longer.[12]

So in a couple billion years, our planet will be much like Venus.

 

[SLD]

One billion years from now, about 27% of the modern ocean will have been subducted into the mantle. If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the current surface reservoir would remain at the surface.[56] Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C; 116 °F). The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans.[89][90] At this point, models of the Earth's future environment demonstrate that the stratosphere would contain increasing levels of water. These water molecules will be broken down through photodissociation by solar UV, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's seawater by about 1.1 billion years from the present.[91][92]

There will be two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere while water vapor starts to accumulate in the stratosphere (if the oceans evaporate very quickly), and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere (if the oceans evaporate too slowly). In this ocean-free era, there will continue to be surface reservoirs as water is steadily released from the deep crust and mantle,[56] where it is estimated that there is an amount of water equivalent to several times that currently present in the Earth's oceans.[93] Some water may be retained at the poles and there may be occasional rainstorms, but for the most part, the planet would be a desert with large dunefields covering its equator, and a few salt flats on what was once the ocean floor, similar to the ones in the Atacama Desert in Chile.[14]

With no water to serve as a lubricant, plate tectonics would very likely stop and the most visible signs of geological activity would be shield volcanoes located above mantle hotspots.[90][79] In these arid conditions the planet may retain some microbial and possibly even multicellular life.[90] Most of these microbes will be halophiles and life could find refuge in the atmosphere as has been proposed to have happened on Venus.[79] However, the increasingly extreme conditions will likely lead to the extinction of the prokaryotes between 1.6 billion years[88] and 2.8 billion years from now, with the last of them living in residual ponds of water at high latitudes and heights or in caverns with trapped ice. However, underground life could last longer.[12]

So in a couple billion years, our planet will be much like Venus.

Except we can’t predict our orbit that far out, right?

what would happen if a star would come swinging by our solar system enough to cause us to be thrown out of whack such that we would find ourselves in an orbit near the Kuiper belt? Could we technologically develop an underground civilization such that we could survive? We’d have 10,000 years warning.