I`ve recently watched the video about NASA`s the most expensive telescope. James Webb Space Telescope is considered to be the most technological and most advanced space telescope that is made for discovering exoplanets. I guess such project gives us the possibility to study those exoplanets and possibly it will help us find some forms of life on other planets. https://en.wikipedia.org/wiki/James_Web
I'm not quite sure how the James Webb Telescope will be able to detect life, as it has been designed to detect light in a relatively narrow range in the mid-infrared, at wavelengths ranging from 0.6 micrometers to 28.5 micrometers, for the following reasons:
- high-redshift objects which have their visible emissions shifted into the infrared
- cold objects such as debris disks and planets which emit most strongly in the infrared
- this band is difficult to study from the ground or by existing space telescopes such as Hubble
But trying to assess the possibility of extraterrestrial life, it is unknown how abundant such life is across the universe, or whether such life might be complex or intelligent.
On Earth, the emergence of complex intelligent life required a preceding series of evolutionary transitions such as abiogenesis, eukaryogenesis, and the evolution of sexual reproduction, multicellularity, and intelligence itself. Some of these transitions may be seen as extraordinarily improbable, even in conducive environments. The emergence of intelligent life late in Earth's lifetime is thought to be evidence for a handful of rare evolutionary transitions, but the timing of other evolutionary transitions in the fossil record is yet to be analyzed in a similar framework. Using a simplified Bayesian model* that combines uninformative priors and the timing of evolutionary transitions, we demonstrate that expected evolutionary transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude.
The original argument, suggested by Brandon Carter, concerns the idea that intelligent life in the Universe is exceptionally rare, assuming that intelligent life elsewhere requires analogous evolutionary transitions. Arriving at the opposite conclusion would require exceptionally conservative priors, evidence for much earlier transitions, multiple instances of transitions, or an alternative model that can explain why evolutionary transitions took hundreds of millions of years without appealing to rare chance events. Although the model is simple, it provides an initial basis for evaluating how varying biological assumptions and fossil record data impact the probability of evolving intelligent life, and also provides a number of testable predictions, such as that some biological paradoxes will remain unresolved and that planets orbiting M dwarf stars are uninhabitable.
One of the oldest arguments against SETI is the biological contingency argument (Simpson, 1964
): the evolution of anything similar to humans has a minuscule probability since biological evolution is dominated by contingency, is radically open-ended, and has no determinism or tendency toward intelligence. Even in similar environments, the chance of getting “humanoids” is minimal, and most environments will be vastly different. This is the same argument used by Mayr in his debate with Sagan: out of the approximately 50 billion species on Earth, only humans evolved intelligence, suggesting a low probability (Mayr, 1995a, 1995b, 1995c).
Carl Sagan (1995
) countered by noting that if there are enough possible pathways, even individually very unlikely paths can in sum give a high probability of an intelligent outcome. He also noted that extrapolating from our case is either valid, and we should expect Earth to be an average sample, or it is improper to extrapolate, in which case Mayr's argument fails. While the biological contingency argument can be attacked in other ways, for example, by emphasizing convergent evolution (Puccetti, 1968; Morris, 2003), and supported by noting the lack
of convergent evolution toward human-like intelligence in the fossil record (Lineweaver, 2009), the key issue is how representative the Earth's biosphere history is (Rospars, 2013
Brandon Carter proposed a simple model of evolutionary transitions to describe the process of intelligent life emerging. The model proposes that intelligent life requires n
“critical steps,” each of which occurs at some rate λ. He further stipulates that λ−1 > τ⊙, so that the probability per unit time of the critical step is low enough that the time it takes for each critical step will typically exceed the lifetime of the star. A number of interesting properties follow from this model. First, the probability that the final transition occurs at time t
is proportional to tn
, so that the final critical step is likely to occur toward the end of habitable time remaining. Second, the amount of time remaining will be roughly equal to τ⊙/(n
+ 1), allowing one to estimate the number of critical steps that occurred in Earth's evolutionary history simply by knowing the amount of time left in Earth's habitable lifetime.
When Carter originally proposed the model, it was thought that the biosphere could last for another 4 billion years, which in turn suggested that there were likely only one or two critical steps in our evolutionary history. Subsequent improvements in climate models led to additional research that suggested that the time remaining is substantially shorter, on the order of 1 billion years (Caldeira and Kasting, 1992
). A number of researchers have returned to Carter's critical step model and re-estimated the number of critical steps predicted by the remaining lifetime of the biosphere. Watson (2008
) found that the best fit was with four critical steps, while Carter (2008) suggested between five and six. Waltham (2017
) went further to demonstrate that models up to 12 critical steps still fall within a 95% confidence interval. Using the Carter model without further hard steps [e.g.
, just abiogenesis, as in Lineweaver et al
. (2002) and Spiegel and Turner (2012
)] produces significantly different estimates from including hard steps (Flambaum, 2003). The hard step model can also be combined with estimates of the window length (Lingam and Loeb, 2019
), or even possible early windows for abiogenesis that later close (Lineweaver and Davis, 2003
There are a number of reasons why the probability of an evolutionary transition could change over time. Perhaps most importantly, some evolutionary transitions may have required high oxygen concentrations as a source of energy, and oxygen concentrations have changed dramatically over Earth's history (Holland, 2006
). The fact that oxygen concentrations have became high enough to support humans only in the past 800 Myr or so has led to some speculation that a planetary oxygenation time is the primary rate-limiting step to intelligent life (Catling et al.
). Relatedly, complex life on land requires shielding from ultraviolet radiation, and the emergence of an ozone layer has also been hypothesized to be a rate-limiting step that is correlated with stellar evolution, undermining Carter's original argument (Livio, 1999).
To test this, we adjust our model so that the transition rates change over time. The most dramatic example of this is a model in which the final evolutionary transition to intelligent life has a probability of zero until vertebrates on land emerge (340 million years ago), and that transition has probability zero until Phanerozoic oxygen concentrations are reached (800 million years ago). This model essentially tells us that these transitions occurred fairly rapidly once oxygen concentrations were high enough, and the results show a much larger peak around fast rates, suggesting a higher probability of intelligent life emerging in the right conditions. However, even these faster transition times are not enough to exclude extremely slow rates. Overall, accounting for a changing environment in terms of oxygen concentrations does not seem to be sufficient to overturn our key results.
Posterior distribution if we assume two transitions that were made possible only after high oxygenation levels. Given the late oxygenation of Earth's atmosphere, these transition times are short, resulting in higher posterior probability on faster rates. However, arbitrarily slow rates are still not excluded. (Bottom) Posterior distribution if we adopt the self-indication assumption, and weight all parameter combinations by their probability of obtaining intelligent life. Only parameters that are consistent with intelligent life are assigned high probability, and extremely slow rates are ruled out entirely. Color images are available online.
It took approximately 4.5 billion years for a series of evolutionary transitions resulting in intelligent life to unfold on Earth. In another billion years, the increasing luminosity of the Sun will make Earth uninhabitable for complex life. Intelligence therefore emerged late in Earth's lifetime. Together with the dispersed timing of key evolutionary transitions and plausible priors, one can conclude that the expected transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude. In turn, this suggests that intelligent life is likely to be exceptionally rare. Arriving at an alternative conclusion would require either exceptionally conservative priors, finding additional instances of evolutionary transitions, or adopting an alternative model that can explain why evolutionary transitions took so long on Earth without appealing to rare stochastic occurrences. There are a number of other testable predictions, including that M dwarf stars are uninhabitable, that many biological paradoxes will remain unsolved without allowing for extremely unlikely events, and that, counterintuitively, we might be slightly more likely to find simple life on Mars.
We can conclude that intelligent life is exceptionally rare and that we may possibly be the only intelligent civilization within the observable universe, so long as we assume that intelligent life elsewhere requires similar evolutionary transitions. Although this may seem like a large assumption, there are good reasons to believe that many evolutionary transitions have universal properties (Levin et al.
). It also follows if we reason that our civilization is typical. If there were substantially easier evolutionary pathways to intelligent life that did not require such evolutionary transitions, we should expect to observe this easier evolutionary history instead. Although it is hard to show beyond doubt the absence of extraterrestrial intelligence, so far all of our astronomical data are consistent with being alone (Tipler, 1980
* Bayesian statistics is a particular approach to applying probability to statistical problems. It provides us with mathematical tools to update our beliefs about random events in light of seeing new data or evidence about those events
In particular Bayesian inference interprets probability
as a measure of believability
that an individual
may possess about the occurance of a particular event.
We may have a prior
belief about an event, but our beliefs are likely to change when new evidence is brought to light. Bayesian statistics gives us a solid mathematical means of incorporating our prior beliefs, and evidence, to produce new posterior
Bayesian statistics provides us with mathematical tools to rationally update our subjective beliefs in light of new data or evidence.
This is in contrast to another form of statistical inference
, known as classical
statistics, which assumes that probabilities are the frequency
of particular random events occuring in a long run
of repeated trials
A Bayesian analysis of transition times
The objective is to estimate evolutionary transition rates, given how long it took to complete each transition. This can be found by using a Bayesian update as follows:
is the sequence of transition times t
denotes our β
parameters, and P
(β) is a prior density over the expected transition time parameters. The term P
|β), the probability of observing transition times t
given the parameters β, is equivalent to the likelihood function as follows:
However, this likelihood function needs to be renormalized to account for the fact that we can only observe these data if all evolutionary transitions occurred before the end of Earth's lifetime. Accounting for this sample bias can be done by dividing the likelihood L
(β|t) by the probability that all transitions occurred within the lifetime of Earth. If L
is the lifetime of Earth, then our adjusted likelihood function is as follows:
is the probability that all transitions occur before the end of Earth's lifetime.
We have come a long way from Frank Drake's original equation on the probability of extraterrestrial intelligence. Which is the following:
= the number of civilizations
in our galaxy with which communication might be possible (i.e. which are on our current and past light cone
) and where:
∗ = the average rate of star formation
in our galaxy
p = the fraction of those stars that have planets
e = the average number of planets that can potentially support life
per star that has planets
l = the fraction of planets that could support life that actually develop life at some point
i = the fraction of planets with life that actually go on to develop intelligent
c = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
= the length of time for which such civilizations release detectable signals into space
The Andromeda Galaxy. Image credit: Ivan Bok / CC BY 4.0.
When we look up in the night sky and the Andromeda Galaxy, M31 or NGC 224, swims into view in our field glasses, will someone on a planet in one of the habitable zones orbiting a stable G 2 star like our sun be looking back at the Milky Way, or whatever they call our home, wondering the same thing? Will they also have a forum where they can question the many possible pathways life has taken from RNA to intelligence contemplating the cosmos? Or are we, as I fear and they might also, alone.