"Panspermia" ("seeds everywhere") is the term used for the concept that life in various forms is widespread throughout the cosmos, and that extremophiles can survive transit through space to colonize new worlds.
The "Cambrian Explosion" occurred about 500 million years ago, when multicellular life skyrocketed on earth. Prior to that time, life on this planet consisted almost entirely of single-celled or colonial organisms. During this 20-million year period most of the lines of animals appeared, with major diversifications and accelerated complexity.
Those two concepts are discussed in the most interesting scientific review article I've read all month. Herewith some excerpts from the longread:
"... we discuss the recent phylogenetic data which date the emergence of the complex retroviruses of vertebrate lines at or just before the Cambrian Explosion of ∼500 Ma (the widely agreed epochal event in the evolutionary history of multicellular life on Earth). These types of reverse transcribing and genome integrating viruses are speculated to be plausibly associated with major evolutionary genomic processes. We believe this coincidence with the Cambrian Explosion may not be fortuitous...Note: Appendix A to the paper discusses the theory of panspermia as it relates to the existence of a deity as ultimate progenitor for the creation of life.
... life was seeded here on Earth by life-bearing comets as soon as conditions on Earth allowed it to flourish (at or just before 4.1 Billion years ago); and living organisms such as space-resistant and space-hardy bacteria, viruses, more complex eukaryotic cells and organisms (e.g. Tardigrades), perhaps even fertilised ova and plant seeds, may have been continuously delivered ever since to Earth helping to drive further the progress of terrestrial biological evolution...
Even if we concede that the dominant neo-Darwinian paradigm of natural selection can explain aspects of the evolutionary history of life once life gets started, independent abiogenesis on the cosmologically diminutive scale of oceans, lakes or hydrothermal vents remains a hypothesis with no empirical support and is moreover unnecessary and redundant...
... direct evidence of liquid water in comets as well as other icy solar system bodies came to be firmly established through space exploration. The Jovian moon Europa, the Saturnian moon Enceladus and the dwarf planet Ceres all have evidence of liquid water, maintained either through tidal energy dissipation or radioactive heating. ..
It is now becoming amply clear that Earth-like planets and other life-friendly planetary bodies exist in their hundreds of billions...
Since 1980 the existence in interstellar clouds of complex organic molecules such as polycyclic aromatic hydrocarbons, is beyond dispute...
Data from cometary studies continue to be backed up by recoveries of microbial material in the stratosphere (under conditions where upwelling terrestrial contamination can be plausibly ruled out)...
We should then plausibly view viruses as among the most information-rich natural systems in the known Universe. Their size dictates they are very small targets minimizing the probability of destruction by flash heating or ionizing radiation... Their nanometer dimensions plausibly allow easy transport and dispersal by micrometer sized dust grains and other protective physical matrices of similar size. They are then nanoparticle-sized genetic vectors which contain all the essential information to take over and drive the physiology of any given target cell within which they mesh. Their replicative growth means they are produced, and exist, in huge numbers on cosmic scales; so that they (and to a lesser quantitative extent their cellular reservoirs) can suffer huge losses by inactivation while still leaving a residue of millions of surviving particles potentially still infective. A virus then is a type of compressed module in touch with the whole of the cell's very ability to grow and divide to produce progeny cells and thus to evolve...
Evidence of the role of extraterrestrial viruses in affecting terrestrial evolution has recently been plausibly implied in the gene and transcriptome sequencing of Cephalopods. The genome of the Octopus shows a staggering level of complexity with 33,000 protein-coding genes more than is present in Homo sapiens ...
Thus the possibility that cryopreserved Squid and/or Octopus eggs, arrived in icy bolides several hundred million years ago should not be discounted (below) as that would be a parsimonious cosmic explanation for the Octopus' sudden emergence on Earth ca. 270 million years ago. Indeed this principle applies to the sudden appearance in the fossil record of pretty well all major life forms, covered in the prescient concept of “punctuated equilibrium”...
This now leads us to the crux and an important take home lesson of this review. While all viruses, when looked at closely, are exceedingly clever, the Retroviruses (family Retroviridiae) are up there with the most sophisticated and compact of all known viruses. These viruses and their elements (reverse transcriptase enzymes, associated with induced mobile retro-elements) now appear to be important viral-drivers of major evolutionary genetic change on Earth over the past few hundred million years ...
It is well known that a mass extinction event, or events, occurred at the end of the Ediacaran period about 542 million years ago. This was the immediate forerunner of the Cambrian explosion and the mass extinction scale suggests the passage of our Solar System through a Giant Molecular Cloud dislodging multiple long period Oort Cloud comets into the inner Solar System setting up impacts with the Earth... It takes little imagination to consider that the pre-Cambrian mass extinction event(s) was correlated with the impact of a giant life-bearing comet (or comets), and the subsequent seeding of Earth with new cosmic-derived cellular organisms and viral genes...
It goes without saying that Tardigrades, micro-segmented tiny eukaryotic animals, which emerged in the Cambrian period pose a serious challenge to traditional neo-Darwinian thinking...
Top image via The Carbon Pilgrim. Bottom image via Yale Scientific.
How does this work in theory? A planet explodes and sends bits off into the universe? Or gets nailed by a comet so large chunks of planet are thrown into space?
ReplyDeleteExplosions would be something from Star Wars, but impacts would not be unexpected. Our moon, for example ("The most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.")
DeleteOn a more reasonable size- and time-scale, some of the meteorites that scientists have been finding while walking around in Antarctica appear to have been ejected from Mars -
https://phys.org/news/2016-04-antarctica-plenty-mars-samples.html
Also bits of the moon have been found there.
As much as I would love to believe in panspermia, some statements in that article are very misleading or even wrong. Examples:
ReplyDelete"abiogenesis on the cosmologically diminutive scale of oceans, lakes or hydrothermal vents remains a hypothesis with no empirical support and is moreover unnecessary and redundant." - Even if life didn't start on earth, it had to start somewhere. To state that abiogenesis is unnecessary and redundant is not very objective.
"They [viruses] are then nanoparticle-sized genetic vectors which contain all the essential information to take over and drive the physiology of any given target cell within which they mesh." - Wrong. Our cells are perfectly safe from the vast majority of viruses on our own world. We can inject ourselves with millions of bacteriophages and nothing happens, and the same is true vice versa. It is extremely, extremely, unlikely that a completely alien virus could simply interact with earth life. For all we know it will not even have the same DNA, if any.
Simple statistics in the vast emptyness of space make it extermely unlikely for any object, flung randomly from a source outside of our solar system, to hit earth. Even if, this object would have traveled for hundreds of thousands of years to get to us. This would be more than enough time for space radiation and other harsh conditions to completely destroy even the hardiest virus or tardigrade...
I can imagine life spreading inside our solar system, but given the age of earth relative to the other planets in our system that doesn't change much about the theories on how life much have emerged.
I think you must have read my excerpts, not the article per se.
Delete"abiogenesis on the cosmologically diminutive scale of oceans, lakes or hydrothermal vents remains a hypothesis with no empirical support and is moreover unnecessary and redundant." - Even if life didn't start on earth, it had to start somewhere. To state that abiogenesis is unnecessary and redundant is not very objective.
Your second sentence is exactly what the authors say in their manuscript. Life (probably) had to start from nothing somewhere and that genesis is multi-trillion times more likely in the cosmos as a whole than it is in the "diminutive scale" of Earth. That's why "abiogenesis" (starting from non-life) on Earth is "unnecessary" to postulate - because is makes more sense to postulate it happening elsewhere, and would be redundant of a process that had occurred elsewhere. And of course despite many scientists best efforts, there is not empirical support for abiogenesis having happened on Earth.
"Simple statistics in the vast emptyness of space make it extermely unlikely for any object, flung randomly from a source outside of our solar system, to hit earth."
DeleteLow probabilities laugh in the face of deep time. In fact, earth HAS been hit countless times by objects from "outer space." The Chicxulub crater would be the most massive known, but uncounted others have been covered by sediments from erosion. To get an idea of how frequently Earth must have been hit, look at the almost-contiguous craters of the (much-smaller) moon.
Ok, indeed I did only read you excerpts, but I stand by the rareness of deep space asteroid hits and the improbability of life surviving the trip. The vast majority of craters on the moon (and earth) are from objects within our solar system.
DeleteComplete bunk, I'm sorry to say.
ReplyDeleteHere's a detailed takedown of the cephalpod piece of the paper specifically: https://fistfulofcinctans.wordpress.com/2018/05/15/the-pseudoscience-of-octopuses-from-space/
In more general terms, There's a lot of extraordinary claims presented as basic knowledge without any mention of the supporting evidence, such as:
"Another related phenomenon concerns the red rain events recorded throughout history (McCafferty, 2008) but most recently in Kerala, India (Louis and Kumar 2006) and Sri Lanka in 2012. All the available evidence point to red pigmented organisms that are unlikely to have a terrestrial origin."
Louis and Kumar (2006), incidentally, give correlation in timing with a possible meteor airburst and the fact that the red substance in the rain looked like cells but had no DNA or RNA (other studies on the same rain did find DNA) as their only evidence for an extraterrestrial origin. McCafferty (2008) is about the same.
There's also a suspicious degree of self-citation for a wide range of vital claims. There's a legitimate time for a lot of self-citation - I'm currently doing scientific research in a sub-sub-subfield where there's maybe six people working and they are all citing each other and themselves - but this is still sending up red flags. It looks like the authors are just citing themselves for very bold claims that their argument relies on because there isn't any external corroboration to cite. This is a good example:
"The average distance between habitable planets in our galaxy now to be reckoned in light years – typically 5 light years (Wickramasinghe et al., 2012)."
because this is a claim I have not seen anywhere else, has very broad relevance in the field of exoplanet research, is very different than every other claim I have seen in the field of exoplanet research, is critical to the argument being made, and is being cited from a paper where three of the five authors are also authors on this paper.
A few citations like this wouldn't be worrying because researchers do build on their previous work and sometimes they are the ones writing the groundbreaking paper that demonstrates this bold claim. It’s the pattern throughout the paper that’s concerning.
There is also a lot of acting as if "we shouldn't rule out the possibility" is a meaningful scientific argument, as seen in:
"Thus the possibility that cryopreserved Squid and/or Octopus eggs, arrived in icy bolides several hundred million years ago should not be discounted (below) as that would be a parsimonious cosmic explanation for the Octopus' sudden emergence on Earth ca. 270 million years ago."
There is no evidence presented for how squid or octopus eggs could survive and be viable after being frozen, transported through space for indefinite periods of time, and then subjected to entry of the meteorite carrying them into Earth's atmosphere. There's certainly no evidence presented for assuming that that did happen, even if it could. If Steele et al. (2018) don't want that possibility to be discounted, then the onus is on them to provide at least some of that evidence.
The idea that octopuses arriving from space is simpler by Occam's Razor than octopuses not coming from space is also bizarre, and a pretty transparent attempt to cram the evolutionary biology jargon "parsimony" in somewhere for legitimacy.
The defensive attitude against "mainstream science" and constant reminders of cases where theories that seemed crazy turned out to be right is also often a sign of somebody who has a poor argument and knows it. If they actually had the solid evidence to overturn the standing scientific paradigm, they would probably not be wasting paragraphs and paragraphs on "this sounds crazy, but...". (Especially given that this is fairly long for a scientific paper and a lot of that space is this kind of bletherskate rather than the actual science.)
"Bletherskate/blatherskate/blitherskate" is a new word for me. Apparently related in etymology (of its second part) to the more familiar "cheapskate." Thank you.
DeleteAnd you make some valid points that I won't try to argue against.
I came here to say what Anonymous explained way better than I could have.
ReplyDeleteThis is not the first time this idea is put forward, always by the same people. https://freethoughtblogs.com/pharyngula/2015/08/14/friday-cephalopod-its-not-really-an-alien/