Rosa Rubicondior: Creationism in Crisis - Scientists Are Questioning Human Evolution - But Not In The Way Creationists Had Hoped

A new model upends the decades-old “hard steps” theory that intelligent life was an incredibly improbable event and suggests that maybe it wasn't all that hard or improbable. The team of researchers said the new interpretation of humanity’s origin increases the probability of intelligent life elsewhere in the universe.

Does planetary evolution favor human-like life? Study ups odds we’re not alone | Penn State University

An interdisciplinary team of astrophysicists and geoscientists has questioned the evolution of a human-like intelligent species. But the question is not about whether, as creationists have been predicting for 50 plus years, but just how easy or difficult was it? That we evolved was never in any doubt.

The relevance of the easy/difficult question is that it affects our calculation of the probability of similar intelligent life evolving on other planets. If each step in its evolution was hard (the 'hard step' model), then we may be alone in the Universe; if easy, then the Universe could be teeming with life.

How is the probability of intelligent life evolving on other planets in the cosmos calculated? The probability of intelligent life evolving on other planets is a complex and uncertain estimate that depends on many variables, most of which are poorly constrained. However, scientists approach this question using probabilistic models, the most famous of which is the **Drake Equation**.

The Drake Equation
Proposed by Frank Drake in 1961, the equation attempts to estimate the number of civilizations in our galaxy with which we might communicate:
\[ N = R_* \times f_p \times n_e \times f_l \times f_i \times f_c \times L \] where:

\( R_* \) = Rate of star formation in our galaxy
\( f_p \) = Fraction of those stars with planets
\( n_e \) = Average number of Earth-like planets per planetary system
\( f_l \) = Fraction of those planets where life actually arises
\( f_i \) = Fraction of planets with life where intelligent life evolves
\( f_c \) = Fraction of intelligent species that develop detectable technology (e.g., radio signals)
\( L \) = The average lifespan of such civilizations
The biggest uncertainties are in \( f_l \), \( f_i \), and \( f_c \). While exoplanet studies have constrained some of the early factors, the emergence of life and intelligence is still poorly understood.

Bayesian Approaches
Some scientists use Bayesian probability models, incorporating our only known example—Earth—to estimate the likelihood of life and intelligence emerging elsewhere. These approaches often make assumptions based on evolutionary biology, chemistry, and the available exoplanet data.

Evolutionary and Chemical Constraints
Life's emergence depends on:

The availability of liquid water and essential organic molecules
The stability of planetary conditions over billions of years
Evolutionary pressures that may favor intelligence
Bottlenecks (e.g., mass extinctions, planetary catastrophes) that can limit the development of intelligence

The Great Filter Hypothesis
Some theories suggest that the probability of intelligent life is low due to one or more "filters" that make it unlikely, such as:

Life itself being rare (abiogenesis is difficult)
Complex multicellular life being rare
Intelligence not being a strong evolutionary advantage
Civilizations destroying themselves before they can communicate

The Anthropic Principle
Some argue that our presence as an intelligent species biases our perspective—we observe intelligence because we are intelligent. This makes estimating probabilities tricky since we don't know how typical our case is.

Statistical Estimates

Optimistic estimates (e.g., Carl Sagan's) suggest thousands of civilizations in the Milky Way.
Pessimistic estimates (e.g., the Rare Earth hypothesis) suggest that Earth-like conditions are extremely rare, making intelligent life exceedingly unlikely.

The team, consisting of Professor Jennifer L. Macalady and Professor Jason T. Wright of Penn State University Extraterrestrial Intelligence Center, Daniel B. Mills of Department of Earth and Environmental Sciences, Palaeontology and Geobiology, Ludwig-Maximilians-Universität München, Germany, and Adam Frank of Department of Physics and Astronomy, University of Rochester, Rochester, NY, USA, have reassessed the 'hard steps' model.

They have concluded that, rather than the steps being hard and depending on unlikely probabilities, they were actually fairly easy if not inevitable and depended on the planet reaching 'windows of opportunity' after which the next major step was determined by the opportunities now available. For example, Earth was initially inhospitable to multicellular life until photosynthesis had evolved, and oxygen became available, creating the conditions for more complex life to evolve.

Their findings are published, open access, in a technical paper in Science Advance and a press release from Penn State University:

Does planetary evolution favor human-like life? Study ups odds we’re not alone
New theory proposes that humans — and analogous life beyond Earth — may represent the probable outcome of biological and planetary evolution
Humanity may not be extraordinary but rather the natural evolutionary outcome for our planet and likely others, according to a new model for how intelligent life developed on Earth.

The model, which upends the decades-old “hard steps” theory that intelligent life was an incredibly improbable event, suggests that maybe it wasn't all that hard or improbable. A team of researchers at Penn State, who led the work, said the new interpretation of humanity’s origin increases the probability of intelligent life elsewhere in the universe.

This is a significant shift in how we think about the history of life. It suggests that the evolution of complex life may be less about luck and more about the interplay between life and its environment, opening up exciting new avenues of research in our quest to understand our origins and our place in the universe.

Professor Jennifer L. Macalady, co-author.
The Penn State Extraterrestrial Intelligence Center
Penn State University, PA, USA.
Initially developed by theoretical physicist Brandon Carter in 1983, the “hard steps” model argues that our evolutionary origin was highly unlikely due to the time it took for humans to evolve on Earth relative to the total lifespan of the sun — and therefore the likelihood of human-like beings beyond Earth is extremely low.

In the new study, a team of researchers that included astrophysicists and geobiologists argued that Earth's environment was initially inhospitable to many forms of life, and that key evolutionary steps only became possible when the global environment reached a "permissive" state.

For example, complex animal life requires a certain level of oxygen in the atmosphere, so the oxygenation of Earth’s atmosphere through photosynthesizing microbes and bacteria was a natural evolutionary step for the planet, which created a window of opportunity for more recent life forms to develop, explained Dan Mills, postdoctoral researcher at The University of Munich and lead author on the paper.

We're arguing that intelligent life may not require a series of lucky breaks to exist. Humans didn't evolve ‘early’ or ‘late’ in Earth’s history, but ‘on time,’ when the conditions were in place. Perhaps it’s only a matter of time, and maybe other planets are able to achieve these conditions more rapidly than Earth did, while other planets might take even longer.

Daniel B. Mills, first author
Department of Earth and Environmental Sciences, Paleontology and Geobiology
Ludwig-Maximilians-Universität München, Germany.
And The Penn State Extraterrestrial Intelligence Center
Penn State, University Park, PA, USA.
The central prediction of the “hard steps” theory states that very few, if any, other civilizations exist throughout the universe, because steps such as the origin of life, the development of complex cells and the emergence of human intelligence are improbable based on Carter’s interpretation of the sun's total lifespan being 10 billion years, and the Earth's age of around 5 billion years.

In the new study, the researchers proposed that the timing of human origins can be explained by the sequential opening of “windows of habitability” over Earth's history, driven by changes in nutrient availability, sea surface temperature, ocean salinity levels and the amount of oxygen in the atmosphere. Given all the interplaying factors, they said, the Earth has only recently become hospitable to humanity — it’s simply the natural result of those conditions at work.

We’re taking the view that rather than base our predictions on the lifespan of the sun, we should use a geological time scale, because that's how long it takes for the atmosphere and landscape to change. These are normal timescales on the Earth. If life evolves with the planet, then it will evolve on a planetary time scale at a planetary pace.

Professor Jason T. Wright, co-author
The Penn State Extraterrestrial Intelligence Center
Penn State, University Park, PA, USA.
Wright explained that part of the reason that the “hard steps” model has prevailed for so long is that it originated from his own discipline of astrophysics, which is the default field used to understand the formation of planets and celestial systems. The team’s paper is a collaboration between physicists and geobiologists, each learning from each other’s fields to develop a nuanced picture of how life evolves on a planet like Earth.

This paper is the most generous act of interdisciplinary work. Our fields were far apart, and we put them on the same page to get at this question of how we got here and are we alone? There was a gulf, and we built a bridge.

Professor Jennifer L. Macalady.

The researchers said they plan to test their alternative model, including questioning the unique status of the proposed evolutionary “hard steps.” The recommended research projects are outlined in the current paper and include such work as searching the atmospheres of planets outside our solar system for biosignatures, like the presence of oxygen. The team also proposed testing the requirements for proposed “hard steps” to determine how hard they actually are by studying uni- and multicellular forms of life under specific environmental conditions such as lower oxygen and temperature levels.

Beyond the proposed projects, the team suggested the research community should investigate whether innovations —such as the origin of life, oxygenic photosynthesis, eukaryotic cells, animal multicellularity and Homo sapiens — are truly singular events in Earth's history. Could similar innovations have evolved independently in the past, but evidence that they happened was lost due to extinction or other factors?

This new perspective suggests that the emergence of intelligent life might not be such a long shot after all. Instead of a series of improbable events, evolution may be more of a predictable process, unfolding as global conditions allow. Our framework applies not only to Earth, but also other planets, increasing the possibility that life similar to ours could exist elsewhere.

Professor Jason T. Wright.

The other co-author on the paper is Adam Frank of the University of Rochester. Penn State’s Astrobiology Research Center, the Penn State Center for Exoplanets and Habitable Worlds, the Penn State Extraterrestrial Intelligence Center, the NASA Exobiology program and the German Research Foundation supported this work.

Abstract
According to the “hard-steps” model, the origin of humanity required “successful passage through a number of intermediate steps” (so-called “hard steps”) that were intrinsically improbable in the time available for biological evolution on Earth. This model similarly predicts that technological life analogous to human life on Earth is “exceedingly rare” in the Universe. Here, we critically reevaluate core assumptions of the hard-steps model through the lens of historical geobiology. Specifically, we propose an alternative model where there are no hard steps, and evolutionary singularities required for human origins can be explained via mechanisms outside of intrinsic improbability. Furthermore, if Earth’s surface environment was initially inhospitable not only to human life, but also to certain key intermediate steps required for human existence, then the timing of human origins was controlled by the sequential opening of new global environmental windows of habitability over Earth history.

INTRODUCTION—WHAT IS THE “HARD-STEPS” MODEL?
In 1983, the physicist B. Carter concluded that the time it took for humans to evolve on Earth (relative to the total lifespan of the Sun) suggests that our evolutionary origin was intrinsically unlikely and that comparable human-like observers beyond Earth are rare (1). Carter arrived at this conclusion by noting the order-of-magnitude coincidence (within a factor of about 2) between the age of Earth as it now appears to us (\(t_e\) ≈ 0.5 × 1010 years, roughly equivalent to the timing of our emergence) and the estimated main sequence lifespan of the Sun \(\tau_0\) = 1010 years, corresponding roughly to the habitable lifetime of Earth). If one assumes, as Carter did, that the biological processes dictating evolutionary timescales on Earth and the physical processes determining the main sequence lifetime of the Sun “have nothing directly to do with each other” (1), then there is no a priori reason for predicting such a close “observational coincidence” between these two timescales (2). Carter, noting that evolutionary theory is unable to predict the “expected average time” (\(t_i\)) intrinsically required to evolve “intelligent observers,” evaluated the different possibilities relating these three timescales to broadly constrain \(t_i\): (i) \(t_i\) ≪ \(\tau_0\); (ii) \(t_i\) ≈ \(t_e\); and (iii) \(t_i\) ≫ \(\tau_0\). Carter rejected (i) on the grounds that if it were true, then \(t_e\) should have a much smaller value than \(\tau_0\) (we should find ourselves on a much younger Earth), and “it is hard to think of any particular reason why our arrival should have been greatly delayed relative to the intrinsically expected time [\(t_i\)]” (1). Next, Carter rejected (ii) as “much less plausible a priori” than the alternatives, and recommended considering it only if convincing a posteriori evidence against the two remaining possibilities were to arise (1). Carter ultimately settled on (iii) arguing that if one accepts \(t_i\) >> \(\tau_0\), then \(t_e\) ≈ \(\tau_0\) becomes explicable—indeed expected—by applying the self-selection (or “anthropic”) principle (3), in that if we are going to evolve, we must necessarily evolve before \(\tau_0\) (2), and on timescales most probably approaching \(\tau_0\). It is through the application of this anthropic reasoning that Carter predicted that our evolutionary origin was inherently improbable within the externally allotted time (\(\tau_0\)) (4), with the corollary that analogous intelligent observers beyond Earth would be equally improbable.

To explain the inherent unlikelihood of human origins, Carter proposed that the evolutionary emergence of humans must have depended on the “successful passage through a number of intermediate steps” in which traits necessary for human existence were gained (1). If the mean time required for such an essential step is “small” relative to \(\tau_0\), then Carter considered the step “easy” (2), happening “effectively deterministically” (2) with “virtual certainty” in the provided time (\(\tau_0\)) (1). However, if the intrinsic mean time required for an essential intermediate step is “large” compared to \(\tau_0\) (2)—“at least a significant fraction” of \(\tau_0\) (1)—then Carter variously considered such a step “critical” (1, 5), “difficult” (2), or “hard” (6). Because “easy” steps are more or less guaranteed to occur with respect to \(\tau_0\), it is only the “hard” steps that need to be considered in estimating the likelihood of human existence within \(\tau_0\). Recognizing that the conditional probability (P) of successfully completing a number of equally unlikely steps (n) within time (t) follows the power law expression \(P\approx t^n\), Carter initially proposed only one or two hard steps (1, 2), as n ≥ 3 predicts that steps would most likely be completed “very near” the upper limit of \(\tau_0\) and that while our sun is “no longer young,” the time remaining in its main sequence lifetime is nevertheless too great to reconcile with more than two steps (2). Favoring this logic, Carter ultimately concluded that at most one or two of the essential steps in our evolutionary history were truly “hard” within the bounds of the Sun’s lifespan, and that the evolution of comparable biological beings on worlds beyond Earth would similarly depend on “chance events with characteristic timescales long compared with those of stellar evolution” (4). With this formalization, the hard-steps model was born.

Since 1983, the hard-steps (or critical-steps or Carter) model has been used and refined by numerous authors (7–14) and remains a pervasive and influential framework for predicting the distribution and complexity of life beyond Earth (15–19). The hard-steps model also inspired the related concept of the “Great Filter” (Box 1), which more explicitly accounts for the claimed lack of evidence surrounding extraterrestrial intelligence (20). Consistent with Carter’s predictions, proponents of the hard-steps model generally reiterate that human beings were an unlikely product of biological evolution on Earth, and that human-like life elsewhere in the observable universe is exceedingly rare. However, various aspects of the hard-steps model have been criticized, particularly the assumption that biospheric evolution unfolds independently of solar evolution (8, 21, 22), as well as the rejection of factors (for example, environmental) that could have conceivably “delayed” our evolutionary emergence in the scenario t_i ≪ \(\tau_0\) (21, 23, 24). Curiously, while these issues concern the evolutionary history of Earth’s biosphere, comparatively few Earth historians (12, 14, 16, 21), and even fewer evolutionary biologists (25), have responded to Carter’s arguments in the literature, having primarily left astrophysicists, economists, and futurists to champion the hard-steps model unchecked. To correct for this historical imbalance, greater attention from the Earth and life sciences is needed to more exhaustively evaluate and test the core tenets of the hard-steps model.
Box 1

The Fermi Paradox, the Great Silence, the Drake Equation, Rare Earth, and the Great Filter.

The so-called “Fermi Paradox” is named after physicist E. Fermi based on a lunchtime conversation in 1950 in which he asked, “where is everybody?” in the context of the recent reports of UFOs that had been linked to alien spacecraft. The essence of Fermi’s question—later formally developed by Hart (181) and Tipler (182) into what would be called the Fermi Paradox—is that the time to cross the Milky Way galaxy, even in slow ships [taking of order 100 million years (Myr)], is much shorter than the age of the galaxy (of order 10,000 Myr). Any spacefaring species has thus had plenty of time to colonize Earth—so why do we see no trace of them here on Earth?

The Fermi Paradox is often conflated with the “Great Silence” (183), which describes the ostensibly puzzling lack of success of SETI to date to find any signs of technological life elsewhere in the Galaxy. This “silence” is purportedly a puzzle because of optimistic numbers one can calculate for our expectation of SETI signals to detect from the Drake Equation, which is a heuristic order-of-magnitude calculation often used to justify SETI efforts. The Drake Equation (184) includes a series of terms for the number of potentially life-hosting planets in the Galaxy, the fraction of those planets that give rise to life, intelligence, and technology, and a term for how long such technological life lasts. [Note that most SETI practitioners do not find the lack of success of SETI puzzling at all, given the limited searching that has been done and the very large search space; see, for instance, Wright et al. (20).]

One “solution” to Fermi’s paradox and explanation for the Great Silence is that the fractions in Drake’s Equation are very small: That is, planets like Earth are so very rare, and the evolutionary contingencies that lead to animal-like life are so very unlikely, that despite the huge number of stars in the Galaxy, Earth is the only planet in the Milky Way with such life. This is called the “Rare Earth” argument, made most forcefully by Ward and Brownlee (185).

Starting from the assumption that the Fermi Paradox and Great Silence are puzzles demanding a solution, Hanson (10) proposed that there exists a “Great Filter”: some very unlikely step or set of steps on the road to the development of Galaxy-spanning spacefaring life that prevents it from arising, despite optimistic estimates one might calculate from the Drake Equation.

Hanson identified nine essential steps to the widespread colonization of the Milky Way. Eight of these are in Earth’s past, including several of the proposed hard steps mentioned in our review here (Table 1 Opens in image viewer), and an additional first step capturing many other Rare Earth terms. Hanson’s ninth and final step is an exponential “colonization explosion” that leads to nearly every stellar system becoming inhabited.

The popular appeal of the Great Filter framework is that it allows one to consider whether the hypothetical Great Filter is “ahead of us” or “behind us.” That is, it is possible that none of the first eight steps is unlikely, and that human-like technology exists throughout the universe but never spreads because there is something in the galaxy that prevents interstellar colonies from taking hold. This could be a reliable form of destruction of all technological species before they become interstellar (such as nuclear war or some other cataclysm). Science fiction also offers the suggestion that it could be an incumbency effect, where another, prior species checks or exterminates any species that attempts to spread beyond its home planet, like a gardener doing weeding or a sort of “galactic immune system.”

By the logic of the Great Filter, then, the discovery of ancient life on Mars would eliminate many of the early Great Filter candidates from the list, increasing the chances that the Filter lies in our future and humanity’s days are numbered. Alternatively, finding that one or more other Solar System bodies have always been sterile would imply the filter might be behind us, and that we will be the first species to colonize the galaxy. As we demonstrate in the main body of the paper, however, substantive arguments exist against the hard-steps model on which the Great Filter depends.
In this paper, we challenge key fundamental assumptions of the hard-steps model through the lens of historical geobiology (26), the study of how Earth’s surface environment and life have coevolved over geologic time. In short, if Earth’s surface environment was initially inhospitable not only to human life, but also to certain key intermediate steps in human evolution (e.g., the origin of eukaryotic cells and the origin of animal multicellularity), then the “delay” in our evolutionary origin can be best explained by the geological timescales necessary for creating the global-environmental conditions required for humans or human analogs. In this view, we find ourselves so close to the upper limit of Earth’s habitability because this is where the geologically narrow “window of human habitability” is located relative to Earth’s total habitable lifespan.

For clarity, while we examine many of the published interpretations of the hard-steps model, as well as many of the published hard-steps candidates, our critique is more fundamental than this. We examine and challenge the original assumptions used to justify the hard-steps model in the first place, questioning whether the model is even necessary for explaining the temporal coincidence between human origins (\(t_e\)) and the predicted extinction of Earth’s biosphere (\(\tau_0\)). Furthermore, we do not claim to have successfully falsified the hard-steps model, but we do claim to have formulated a justifiable and testable alternative to it. While our alternative model proposes that the evolutionary origin of humans or human analogs was more predictable or probable than the hard-steps model concludes, we do not claim that the evolution of Homo sapiens in particular was “inevitable.”

The facts for creationists to ignore here is that firstly, the scientists are in absolutely no doubt that intelligent life was the result of an evolutionary process caused by the interactions between living organisms and their environment. The only question is how much of it was due to random chance and how much of it was due to predictable and expected evolutionary trajectories that converged towards optimal solutions for a given set of environmental conditions leading inexorably to the evolution of sentience, then intelligence as we understand it.

Of course, this is diametrically opposed to the creationist notion of a god creating the right conditions for its ultimate creation – humankind - and just taking hundreds of million years to get there and making it look like an evolutionary process. Or alternatively casting magic spells just a few thousand years ago to conjure up everything, including intelligent human life out of nothing, then forging the evidence to make Earth look old and humans look like the result of a few billion years of evolution.

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