The Limits to Growth

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https://en.wikipedia.org/wiki/The_Limits_to_Growth

The Limits to Growth

First edition cover
Authors	Donella H. Meadows Dennis L. Meadows Jørgen Randers William W. Behrens III
Language	English
Published	2 March 1972; 52 years ago
Publisher	Potomac Associates – Universe Books
Pages	205
ISBN	0-87663-165-0
OCLC	307838

The Limits to Growth (LTG) is a 1972 report that discussed the possibility of exponential economic and population growth with finite supply of resources, studied by computer simulation. The study used the World3 computer model to simulate the consequence of interactions between the Earth and human systems. The model was based on the work of Jay Forrester of MIT as described in his book World Dynamics.

Commissioned by the Club of Rome, the study saw its findings first presented at international gatherings in Moscow and Rio de Janeiro in the summer of 1971. The report's authors are Donella H. Meadows, Dennis L. Meadows, Jørgen Randers, and William W. Behrens III, representing a team of 17 researchers.

The report's findings suggest that, in the absence of significant alterations in resource utilization, it is highly likely that there will be an abrupt and unmanageable decrease in both population and industrial capacity. Although it faced severe criticism and scrutiny upon its release, the report influenced environmental reforms for decades. Subsequent analysis notes that global use of natural resources has been inadequately reformed to alter its expected outcome. Yet price predictions based on resource scarcity failed to materialize in the years since publication.

Since its publication, some 30 million copies of the book in 30 languages have been purchased. It continues to generate debate and has been the subject of several subsequent publications.

Beyond the Limits and The Limits to Growth: The 30-Year Update were published in 1992 and 2004 respectively; in 2012, a 40-year forecast from Jørgen Randers, one of the book's original authors, was published as 2052: A Global Forecast for the Next Forty Years; and in 2022 two of the original Limits to Growth authors, Dennis Meadows and Jørgen Randers, joined 19 other contributors to produce Limits and Beyond.

World3 Model Standard Run as shown in The Limits to Growth

Purpose

In commissioning the MIT team to undertake the project that resulted in LTG, the Club of Rome had three objectives:

1. Gain insights into the limits of our world system and the constraints it puts on human numbers and activity.
2. Identify and study the dominant elements, and their interactions, that influence the long-term behavior of world systems.
3. To warn of the likely outcome of contemporary economic and industrial policies, with a view to influencing changes to a sustainable lifestyle.

Method

The World3 model is based on five variables: "population, food production, industrialization, pollution, and consumption of nonrenewable natural resources". At the time of the study, all these variables were increasing and were assumed to continue to grow exponentially, while the ability of technology to increase resources grew only linearly. The authors intended to explore the possibility of a sustainable feedback pattern that would be achieved by altering growth trends among the five variables under three scenarios. They noted that their projections for the values of the variables in each scenario were predictions "only in the most limited sense of the word", and were only indications of the system's behavioral tendencies. Two of the scenarios saw "overshoot and collapse" of the global system by the mid- to latter-part of the 21st century, while a third scenario resulted in a "stabilized world".

Exponential reserve index

A key idea in The Limits to Growth is the notion that if the rate of resource use is increasing, the number of reserves cannot be calculated by simply taking the current known reserves and dividing them by the current yearly usage, as is typically done to obtain a static index. For example, in 1972, the amount of chromium reserves was 775 million metric tons, of which 1.85 million metric tons were mined annually. The static index is 775/1.85=418 years, but the rate of chromium consumption was growing at 2.6 percent annually, or exponentially. If instead of assuming a constant rate of usage, the assumption of a constant rate of growth of 2.6 percent annually is made, the resource will instead last

${\displaystyle \frac{\ln (1+0.026\times 418)}{0.026}\approx \text{95 years}}$

In general, the formula for calculating the amount of time left for a resource with constant consumption growth is:

${\displaystyle y=\frac{\ln ((rs)+1)}{r}}$

where:

y = years left;

r = the continuous compounding growth rate;

s = R/C or static reserve;

R = reserve;

C = (annual) consumption.

Commodity reserve extrapolation

The chapter contains a large table that spans five pages in total, based on actual geological reserves data for a total of 19 non-renewable resources, and analyzes their reserves at 1972 modeling time of their exhaustion under three scenarios: static (constant growth), exponential, and exponential with reserves multiplied by 5 to account for possible discoveries. A short excerpt from the table is presented below:

		Years
Resource	Consumption, projected average annual growth rate	Static index	Exponential index	5× reserves exponential index
Chromium	2.6%	420	95	154
Gold	4.1%	11	9	29
Iron	1.8%	240	93	173
Lead	2.0%	26	21	64
Petroleum	3.9%	31	20	50

The chapter also contains a detailed computer model of chromium availability with current (as of 1972) and double the known reserves as well as numerous statements on the current increasing price trends for discussed metals:

Given present resources consumption rates and the projected increase in the rates, the great majority of the currently important nonrenewable resources will be extremely costly 100 years from now. (...) The prices of those resources with the shortest static reserve indices have already begun to increase. The price of mercury, for example, has gone up 500 percent in the last 20 years; the price of lead has increased 300 percent in the last 30 years.

— Chapter 2, page 66

Interpretations of the exhaustion model

Due to the detailed nature and use of actual resources and their real-world price trends, the indexes have been interpreted as a prediction of the number of years until the world would "run out" of them, both by environmentalist groups calling for greater conservation and restrictions on use and by skeptics criticizing the accuracy of the predictions. This interpretation has been widely propagated by media and environmental organizations, and authors who, apart from a note about the possibility of the future flows being "more complicated", did not clearly constrain or deny this interpretation.

While environmental organizations used it to support their arguments, a number of economists used it to criticize LTG as a whole shortly after publication in the 1970s (Peter Passel, Marc Roberts, and Leonard Ross), with similar criticism reoccurring from Ronald Baily, George Goodman and others in the 1990s. In 2011 Ugo Bardi in "The Limits to Growth Revisited" argued that "nowhere in the book was it stated that the numbers were supposed to be read as predictions", nonetheless as they were the only tangible numbers referring to actual resources, they were promptly picked as such by both supporters as well as opponents.

While Chapter 2 serves as an introduction to the concept of exponential growth modeling, the actual World3 model uses an abstract "non-renewable resources" component based on static coefficients rather than the actual physical commodities described above.

Conclusions

After reviewing their computer simulations, the research team came to the following conclusions:

1. If the present growth trends in world population, industrialization, pollution, food production, and resource depletion continue unchanged, the limits to growth on this planet will be reached sometime within the next one hundred years. The most probable result will be a rather sudden and uncontrollable decline in both population and industrial capacity.
2. It is possible to alter these growth trends and to establish a condition of ecological and economic stability that is sustainable far into the future. The state of global equilibrium could be designed so that the basic material needs of each person on earth are satisfied and each person has an equal opportunity to realize his individual human potential.
3. If the world's people decide to strive for this second outcome rather than the first, the sooner they begin working to attain it, the greater will be their chances of success.

— Limits to Growth, Introduction

The introduction goes on to say:

These conclusions are so far-reaching and raise so many questions for further study that we are quite frankly overwhelmed by the enormity of the job that must be done. We hope that this book will serve to interest other people, in many fields of study and in many countries of the world, to raise the space and time horizons of their concerns, and to join us in understanding and preparing for a period of great transition – the transition from growth to global equilibrium.

Criticism

See also: Malthusian § Criticism, and Club of Rome § Critics

LTG provoked a wide range of responses, including immediate criticisms almost as soon as it was published.

Peter Passell and two co-authors published a 2 April 1972 article in the New York Times describing LTG as "an empty and misleading work ... best summarized ... as a rediscovery of the oldest maxim of computer science: Garbage In, Garbage Out". Passell found the study's simulation to be simplistic while assigning little value to the role of technological progress in solving the problems of resource depletion, pollution, and food production. They charged that all LTG simulations ended in collapse, predicting the imminent end of irreplaceable resources. Passell also charged that the entire endeavour was motivated by a hidden agenda: to halt growth in its tracks.

In 1973, a group of researchers at the Science Policy Research Unit at the University of Sussex concluded that simulations in Limits to Growth were very sensitive to a few key assumptions and suggest that the MIT assumptions were unduly pessimistic, and the MIT methodology, data, and projections were faulty. However, the LTG team, in a paper entitled "A Response to Sussex", described and analyzed five major areas of disagreement between themselves and the Sussex authors. The team asserted that the Sussex critics applied "micro reasoning to macro problems", and suggested that their own arguments had been either misunderstood or wilfully misrepresented. They pointed out that the critics had failed to suggest any alternative model for the interaction of growth processes and resource availability, and "nor had they described in precise terms the sort of social change and technological advances that they believe would accommodate current growth processes."

During that period, the very idea of any worldwide constraint, as indicated in the study, was met with scepticism and opposition by both businesses and the majority of economists. Critics declared that history proved the projections to be incorrect, such as the predicted resource depletion and associated economic collapse by the end of the 20th century. The methodology, the computer, the conclusions, the rhetoric and the people behind the project were criticised. Yale economist Henry C. Wallich agreed that growth could not continue indefinitely, but that a natural end to growth was preferable to intervention. Wallich stated that technology could solve all the problems the report was concerned about, but only if growth continued apace. According to Wallich's cautionary statement, prematurely halting progress would result in the perpetual impoverishment of billions.

Julian Simon, a professor at the Universities of Illinois and, later, Maryland, argued that the fundamental underlying concepts of the LTG scenarios were faulty because the very idea of what constitutes a "resource" varies over time. For instance, wood was the primary shipbuilding resource until the 1800s, and there were concerns about prospective wood shortages from the 1500s on. But then boats began to be made of iron, later steel, and the shortage issue disappeared. Simon argued in his book The Ultimate Resource that human ingenuity creates new resources as required from the raw materials of the universe. For instance, copper will never "run out". History demonstrates that as it becomes scarcer its price will rise and more will be found, more will be recycled, new techniques will use less of it, and at some point a better substitute will be found for it altogether. His book was revised and reissued in 1996 as The Ultimate Resource 2.

To the US Congress in 1973, Allen V. Kneese and Ronald Riker of Resources for the Future (RFF) testified that in their view, "The authors load their case by letting some things grow exponentially and others not. Population, capital and pollution grow exponentially in all models, but technologies for expanding resources and controlling pollution are permitted to grow, if at all, only in discrete increments." However, their testimony also noted the possibility of "relatively firm long-term limits" associated with carbon dioxide emissions, that humanity might "loose upon itself, or the ecosystem services on which it depends, a disastrously virulent substance", and (implying that population growth in "developing countries" is problematic) that "we don't know what to do about it".

In 1997, the Italian economist Giorgio Nebbia observed that the negative reaction to the LTG study came from at least four sources: those who saw the book as a threat to their business or industry; professional economists, who saw LTG as an uncredentialed encroachment on their professional perquisites; the Catholic church, which bridled at the suggestion that overpopulation was one of mankind's major problems; finally, the political left, which saw the LTG study as a scam by the elites designed to trick workers into believing that a proletarian paradise was a pipe dream. A UK government report found that "In the 1990s, criticism tended to focus on the misconception that Limits to Growth predicted global resource depletion and social collapse by the end of the year 2000".

Peter Taylor and Frederick Buttle’s interpretation of the LTG study and the associated system dynamics (SD) models found that the original SD was created for firms and set the pattern for urban, global, and other SD models. These firm-based SDs relied on superintending managers to prevent undesirable cycling and feedback loops caused by separate common-sense decisions made by individual sectors. However, the later global model lacked superintending managers that enforce interrelated world-level changes, making undesirable cycles and exponential growth and collapse happen in nearly all models no matter the parameter settings. There was no way for a few individuals in the model to override the structure of the system even if they understood the system as a whole. This meant there were only two solutions: convincing everyone in the system to change the basic structure of population growth and collapse (moral response) and/or having a superintending agency analyzing the system as a whole and directing changes (technocratic response). The LTG report combined these two approaches multiple times. System dynamists constructed interventions into the world model to demonstrate how their proposed interventions improved the system to prevent collapse. The SD model also aggregated the world’s population and resources which meant that it did not demonstrate how crises emerge at different times and in different ways without any strictly global logic or form because of the unequal distributions of populations and resources. These issues indicate that the local, national, and regional differentiation in politics and economics surrounding socioenvironmental change was excluded from the SD used by LTG, making it unable to accurately demonstrate real-world dynamics.

Positive reviews

With few exceptions, economics as a discipline has been dominated by a perception of living in an unlimited world, where resource and pollution problems in one area were solved by moving resources or people to other parts. The very hint of any global limitation as suggested in the report The Limits to Growth was met with disbelief and rejection by businesses and most economists. However, this conclusion was mostly based on false premises.

Meyer & Nørgård (2010)

In 1980, the Global 2000 Report to the President arrived at similar conclusions regarding expected global resource scarcity, and the need for multilateral coordination to prepare for this situation.

In a 2008 blog post, Ugo Bardi commented that "Although, by the 1990s LTG had become everyone's laughing stock, among some the LTG ideas are becoming again popular". Reading LTG for the first time in 2000, Matthew Simmons concluded his views on the report by saying, "In hindsight, The Club of Rome turned out to be right. We simply wasted 30 important years ignoring this work."

Robert Solow, who had been a vocal critic of LTG, said in 2009 that "thirty years later, the situation may have changed... it will probably be more important in the future to deal intellectually, quantitatively, as well as practically, with the mutual interdependence of economic growth, natural resource availability, and environmental constraints".

In a study conducted in 2008, Graham Turner from CSIRO discovered a significant correlation between the observed historical data spanning from 1970 to 2000 and the simulated outcomes derived from the "standard run" limits of the growth model. This correlation was apparent across nearly all the reported outputs. The comparison falls comfortably within the range of uncertainty for almost all the available data, both in terms of magnitude and the patterns observed over time. Turner conducted an analysis of many studies, with a special focus on those authored by economists, that have consistently aimed to discredit the limits-to-growth concept over the course of several years. According to Turner, the aforementioned studies exhibit flaws and demonstrate a lack of comprehension regarding the model.

Turner reprised these observations in another opinion piece in The Guardian on 2 September 2014. Turner used data from the UN to claim that the graphs almost exactly matched the 'Standard Run' from 1972 (i.e. the worst-case scenario, assuming that a 'business as usual' attitude was adopted, and there were no modifications of human behaviour in response to the warnings in the report). Birth rates and death rates were both slightly lower than projected, but these two effects cancelled each other out, leaving the growth in world population almost exactly as forecast.

In 2010, Nørgård, Peet and Ragnarsdóttir called the book a "pioneering report", and said that it "has withstood the test of time and, indeed, has only become more relevant."

In 2012, Christian Parenti drew comparisons between the reception of The Limits to Growth and the ongoing global warming controversy. Parenti further remarked that despite its scientific rigour and credibility, the intellectual guardians of influential economic interests actively dismissed LTG as a warning. A parallel narrative is currently unfolding within the realm of climate research.

In 2012, John Scales Avery, a member of the Nobel Prize (1995) winning group associated with the Pugwash Conferences on Science and World Affairs, supported the basic thesis of LTG by stating,

Although the specific predictions of resource availability in Limits to Growth lacked accuracy, its basic thesis – that unlimited economic growth on a finite planet is impossible – was indisputably correct.

Legacy

Updates and symposia

Researchers from China and Indonesia with Dennis Meadows

The Club of Rome has persisted after The Limits to Growth and has generally provided comprehensive updates to the book every five years.

An independent retrospective on the public debate over The Limits to Growth concluded in 1978 that optimistic attitudes had won out, causing a general loss of momentum in the environmental movement. While summarizing a large number of opposing arguments, the article concluded that "scientific arguments for and against each position ... have, it would seem, played only a small part in the general acceptance of alternative perspectives."

In 1989, a symposium was held in Hanover, entitled "Beyond the Limits to Growth: Global Industrial Society, Vision or Nightmare?" and in 1992, Beyond the Limits (BTL) was published as a 20-year update on the original material. It "concluded that two decades of history mainly supported the conclusions we had advanced 20 years earlier. But the 1992 book did offer one major new finding. We suggested in BTL that humanity had already overshot the limits of Earth's support capacity."

Limits to Growth: The 30-Year Update was published in 2004. The authors observed that "It is a sad fact that humanity has largely squandered the past 30 years in futile debates and well-intentioned, but halfhearted, responses to the global ecological challenge. We do not have another 30 years to dither. Much will have to change if the ongoing overshoot is not to be followed by collapse during the twenty-first century."

In 2012, the Smithsonian Institution held a symposium entitled "Perspectives on Limits to Growth". Another symposium was held in the same year by the Volkswagen Foundation, entitled "Already Beyond?"

Limits to Growth did not receive an official update in 2012, but one of its coauthors, Jørgen Randers, published a book, 2052: A Global Forecast for the Next Forty Years.

Comparisons and updated models

In 2008, physicist Graham Turner at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia published a paper called "A Comparison of 'The Limits to Growth' with Thirty Years of Reality". It compared the past thirty years of data with the eleven scenarios laid out in the 1972 book and found that changes in industrial production, food production, and pollution are all congruent with one of the book's eleven scenarios—that of "business as usual". This scenario in Limits points to economic and societal collapse in the 21st century. In 2010, Nørgård, Peet, and Ragnarsdóttir called the book a "pioneering report". They said that, "its approach remains useful and that its conclusions are still surprisingly valid ... unfortunately the report has been largely dismissed by critics as a doomsday prophecy that has not held up to scrutiny."

Also in 2008, researcher Peter A. Victor wrote that even though the Limits team probably underestimated price mechanism's role in adjusting outcomes, their critics have overestimated it. He states that Limits to Growth has had a significant impact on the conception of environmental issues and notes that (in his view) the models in the book were meant to be taken as predictions "only in the most limited sense of the word".

In a 2009 article published in American Scientist entitled Revisiting the Limits to Growth After Peak Oil, Hall and Day noted that "the values predicted by the limits-to-growth model and actual data for 2008 are very close." These findings are consistent with the 2008 CSIRO study which concluded: "The analysis shows that 30 years of historical data compares favorably with key features ... [of the Limits to Growth] "standard run" scenario, which results in collapse of the global system midway through the 21st Century."

In 2011, Ugo Bardi published a book-length academic study of The Limits to Growth, its methods, and historical reception and concluded that "The warnings that we received in 1972 ... are becoming increasingly more worrisome as reality seems to be following closely the curves that the ... scenario had generated." A popular analysis of the accuracy of the report by science writer Richard Heinberg was also published.

In 2012, writing in American Scientist, Brian Hayes stated that the model is "more a polemical tool than a scientific instrument". He went on to say that the graphs generated by the computer program should not, as the authors note, be used as predictions.

In 2014, Turner concluded that "preparing for a collapsing global system could be even more important than trying to avoid collapse." Another 2014 study from the University of Melbourne confirmed that data closely tracked the World3 BAU model.

In 2015, a calibration of the updated World3-03 model using historical data from 1995 to 2012 to better understand the dynamics of today's economic and resource system was undertaken. The results showed that human society has invested more to abate persistent pollution, increase food productivity and have a more productive service sector however the broad trends within Limits to Growth still held true.

In 2016, the UK government established an All-party parliamentary group on Limits to Growth. Its initial report concluded that "there is unsettling evidence that society is still following the 'standard run' of the original study – in which overshoot leads to an eventual collapse of production and living standards". The report also points out that some issues not fully addressed in the original 1972 report, such as climate change, present additional challenges for human development.

In 2020, an analysis by Gaya Herrington, then Director of Sustainability Services of KPMG US, was published in Yale University's Journal of Industrial Ecology. The study assessed whether, given key data known in 2020 about factors important for the "Limits to Growth" report, the original report's conclusions are supported. In particular, the 2020 study examined updated quantitative information about ten factors, namely population, fertility rates, mortality rates, industrial output, food production, services, non-renewable resources, persistent pollution, human welfare, and ecological footprint, and concluded that the "Limits to Growth" prediction is essentially correct in that continued economic growth is unsustainable under a "business as usual" model. The study found that current empirical data is broadly consistent with the 1972 projections and that if major changes to the consumption of resources are not undertaken, economic growth will peak and then rapidly decline by around 2040.

In 2023, the parameters of the World3 model were recalibrated using empirical data up to 2022. This improved parameter set results in a World3 simulation that shows the same overshoot and collapse mode in the coming decade as the original business-as-usual scenario of the Limits to Growth standard run. The main effect of the recalibration update is to raise the peaks of most variables and move them a few years into the future.

Origin of birds

• From Wikipedia, the free encyclopedia
  https://en.wikipedia.org/wiki/Origin_of_birds
  

The Berlin specimen of Archaeopteryx lithographica

The scientific question of within which larger group of animals birds evolved has traditionally been called the "origin of birds". The present scientific consensus is that birds are a group of maniraptoran theropod dinosaurs that originated during the Mesozoic era.

A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird Archaeopteryx in Germany. Birds and extinct non-avian dinosaurs share many unique skeletal traits. Moreover, fossils of more than thirty species of non-avian dinosaur with preserved feathers have been collected. There are even very small dinosaurs, such as Microraptor and Anchiornis, which have long, vaned arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna also shows these long foot feathers. Paleontologist Lawrence Witmer concluded in 2009 that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage. Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastroliths in the digestive system, nest-building, and brooding behaviors.

Although the origin of birds has historically been a contentious topic within evolutionary biology, only a few scientists still dispute the dinosaurian origin of birds, suggesting descent from other types of archosaurian reptiles. Within the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is disputed. The origin of bird flight is a separate but related question for which there are also several proposed answers.

Research history

Huxley, Archaeopteryx and early research

Thomas Henry Huxley (1825–1895)

Scientific investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin's On the Origin of Species. In 1860, a fossilized feather was discovered in Germany's Late Jurassic Solnhofen limestone. Christian Erich Hermann von Meyer described this feather as Archaeopteryx lithographica the next year. Richard Owen described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptiles, including clawed forelimbs and a long, bony tail.

Biologist Thomas Henry Huxley, known as "Darwin's Bulldog" for his tenacious support of the new theory of evolution by means of natural selection, almost immediately seized upon Archaeopteryx as a transitional fossil between birds and reptiles. Starting in 1868, and following earlier suggestions by Carl Gegenbaur, and Edward Drinker Cope, Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon and Compsognathus. The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Like Cope, Huxley proposed an evolutionary relationship between birds and dinosaurs. Although Huxley was opposed by the very influential Owen, his conclusions were accepted by many biologists, including Baron Franz Nopcsa, while others, notably Harry Seeley, argued that the similarities were due to convergent evolution.

Heilmann and the thecodont hypothesis

Main article: The Origin of Birds (book)

A turning point came in the early twentieth century with the writings of Gerhard Heilmann of Denmark. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916, expanding on earlier work by Othenio Abel, published the results of his research in several parts, dealing with the anatomy, embryology, behavior, paleontology, and evolution of birds. His work, originally written in Danish as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds.

Heilmann's hypothetical illustration of a pair of fighting 'Proaves' from 1916

Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds had clavicles (collar bones) fused to form a bone called the furcula ("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in an interpretation of Dollo's law that stated that evolution was not "reversible", Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont" grade of reptiles. Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.

Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book, but these had been misidentified. The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus in 1936. The next report of clavicles in a dinosaur was in a Russian article in 1983.

Contrary to what Heilmann believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischian dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage) have been found in all types of theropods except the most basal ones, Eoraptor and Herrerasaurus. The original report of a furcula in the primitive theropod Segisaurus (1936) was confirmed by a re-examination in 2005. Joined, furcula-like clavicles have also been found in Massospondylus, an Early Jurassic sauropodomorph.

Ostrom, Deinonychus, and the dinosaur renaissance

The similarity of the forelimbs of Deinonychus (left) and Archaeopteryx (right) led John Ostrom to revive the link between dinosaurs and birds.

The tide began to turn against the 'thecodont' hypothesis after the 1964 discovery of a new theropod dinosaur in Montana. In 1969, this dinosaur was described and named Deinonychus by John Ostrom of Yale University. The next year, Ostrom redescribed a specimen of Pterodactylus in the Dutch Teylers Museum as another skeleton of Archaeopteryx. The specimen consisted mainly of a single wing and its description made Ostrom aware of the similarities between the wrists of Archaeopteryx and Deinonychus.

In 1972, British paleontologist Alick Walker hypothesized that birds arose not from 'thecodonts' but from crocodile ancestors like Sphenosuchus. Ostrom's work with both theropods and early birds led him to respond with a series of publications in the mid-1970s in which he laid out the many similarities between birds and theropod dinosaurs, resurrecting the ideas first put forth by Huxley over a century before. Ostrom's recognition of the dinosaurian ancestry of birds, along with other new ideas about dinosaur metabolism, activity levels, and parental care, began what is known as the dinosaur renaissance, which began in the 1960s and, according to some, continues to this day.

Ostrom's revelations also coincided with the increasing adoption of phylogenetic systematics (cladistics), which began in the 1960s with the work of Willi Hennig. Cladistics is an exact method of arranging species based strictly on their evolutionary relationships, which are calculated by determining the evolutionary tree implying the least number of changes in their anatomical characteristics. In the 1980s, cladistic methodology was applied to dinosaur phylogeny for the first time by Jacques Gauthier and others, showing unequivocally that birds were a derived group of theropod dinosaurs. Early analyses suggested that dromaeosaurid theropods like Deinonychus were particularly closely related to birds, a result that has been corroborated many times since.

Feathered dinosaurs in China

Fossil of Sinosauropteryx prima

The early 1990s saw the discovery of spectacularly preserved bird fossils in several Early Cretaceous geological formations in the northeastern Chinese province of Liaoning. In 1996, Chinese paleontologists described Sinosauropteryx as a new genus of bird from the Yixian Formation, but this animal was quickly recognized as a more basal theropod dinosaur closely related to Compsognathus. Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered homologous with the more advanced feathers of birds, although some scientists disagree with this assessment. Chinese and North American scientists described Caudipteryx and Protarchaeopteryx soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully formed feathers closely resembling those of birds. "Archaeoraptor", described without peer review in a 1999 issue of National Geographic, turned out to be a smuggled forgery, but authentic remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian. The morphological gap between non-avian theropods and birds is further closed by the discoveries of extremely bird-like non-avian dinosaurs, as well as non-avian dinosaur-like basal birds.

Digit homology

There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts—a count of the number of phalanges (finger bones) in the hand.

Embryologists and some paleontologists who oppose the bird-dinosaur link have long numbered the digits of birds II-III-IV on the basis of multiple studies of the development in the egg. This is based on the fact that in most amniotes, the first digit to form in a 5-fingered hand is digit IV, which develops a primary axis. Therefore, embryologists have identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. The fossils of advanced theropod (Tetanurae) hands appear to have the digits I-II-III (some genera within Avetheropoda also have a reduced digit IV). If this is true, then the II-III-IV development of digits in birds is an indication against theropod (dinosaur) ancestry. However, with no ontogenical (developmental) basis to definitively state which digits are which on a theropod hand (because no non-avian theropods can be observed growing and developing today), the labelling of the theropod hand is not absolutely conclusive.

Paleontologists have traditionally identified avian digits as I-II-III. They argue that the digits of birds number I-II-III, just as those of theropod dinosaurs do, by the conserved phalangeal formula. The phalangeal count for archosaurs is 2-3-4-5-3; many archosaur lineages have a reduced number of digits, but have the same phalangeal formula in the digits that remain. In other words, paleontologists assert that archosaurs of different lineages tend to lose the same digits when digit loss occurs, from the outside to the inside. The three digits of dromaeosaurs and Archaeopteryx have the same phalangeal formula of I-II-III as digits I-II-III of basal archosaurs. Therefore, the lost digits would be V and IV. If this is true, then modern birds would also possess digits I-II-III. Also, one 1999 publication proposed a frame-shift in the digits of the theropod line leading to birds (thus making digit I into digit II, II to III, and so forth). However, such frame shifts are rare in amniotes and—to be consistent with the theropod origin of birds—would have had to occur solely in the bird-theropod lineage forelimbs and not the hindlimbs (a condition unknown in any animal). This is called Lateral Digit Reduction (LDR) versus Bilateral Digit Reduction (BDR) (see also Limusaurus).

A small minority, known by the acronym BAND (Birds Are Not Dinosaurs), including ornithologists Alan Feduccia and Larry Martin, continues to assert that birds are more closely related to earlier reptiles, such as Longisquama or Euparkeria, than to dinosaurs. Embryological studies of bird developmental biology have raised questions about digit homology in bird and dinosaur forelimbs. However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists.

An alternative to the frame-shift hypothesis is the axis-shift. According to this explanation, the primary limb axis in birds runs through digit III instead of IV. This idea is supported by palaeontological observations, which determine the phalangeal formula 2-3-4-1-X for the last common ancestor of ceratosaurs (including Limusaurus) and tetanurans (including the tridactyl forms with the phalangeal formula 2-3-4-X-X).

Some later embryological data support the identification of bird digits as I, II, III as in their theropod ancestors.

Thermogenic muscle hypothesis

A 2011 publication suggested that selection for the expansion of skeletal muscle, rather than the evolution of flight, was the driving force for the emergence of this clade. Muscles became larger in prospectively endothermic saurians, according to this hypothesis, as a response to the loss of the vertebrate mitochondrial uncoupling protein, UCP1. In mammals, UCP1 functions within brown adipose tissue, which is thermogenic to protect newborns against hypothermia. In modern birds, skeletal muscle serves a similar function and is presumed to have done so in their ancestors. In this view, bipedality and other avian skeletal alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.

Phylogeny

Further information: Coelurosauria, Maniraptora, and Avemetatarsalia

Simplified phylogenetic tree showing the relationship between modern birds and other dinosaurs 

Archaeopteryx has historically been considered the first bird, or Urvogel. Although newer fossil discoveries filled the gap between theropods and Archaeopteryx, as well as the gap between Archaeopteryx and modern birds, phylogenetic taxonomists, in keeping with tradition, almost always use Archaeopteryx as a specifier to help define Aves. Aves has more rarely been defined as a crown group consisting only of modern birds. Nearly all palaeontologists regard birds as coelurosaurian theropod dinosaurs. Within Coelurosauria, multiple cladistic analyses have found support for a clade named Maniraptora, consisting of therizinosauroids, oviraptorosaurs, troodontids, dromaeosaurids, and birds. Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria, which is a sister group to birds (together forming the node-clade Eumaniraptora) within the stem-clade Paraves.

Other studies have proposed alternative phylogenies, in which certain groups of dinosaurs usually considered non-avian may have evolved from avian ancestors. For example, a 2002 analysis found that oviraptorosaurs were basal avians. Alvarezsaurids, known from Asia and the Americas, have been variously classified as basal maniraptorans, paravians, the sister taxon of ornithomimosaurs, as well as specialized early birds. The genus Rahonavis, originally described as an early bird, has been identified as a non-avian dromaeosaurid in several studies. Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it.

Features linking birds and dinosaurs

Many anatomical features are shared by birds and other theropod dinosaurs.

Feathers

Main article: Feathered dinosaur

Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The first specimen was found in the Solnhofen limestone in southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil, with features clearly intermediate between those of non-avian theropod dinosaurs and birds. Discovered just two years after Darwin's seminal Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus.

Parts of a feather

Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. The first of these were initially described as simple filamentous protofeathers, which were reported in dinosaur lineages as primitive as compsognathids and tyrannosauroids. However, feathers indistinguishable from those of modern birds were soon after found in non-avialan dinosaurs as well.

A small minority of researchers have claimed that the simple filamentous "protofeather" structures are simply the result of the decomposition of collagen fiber under the dinosaurs' skin or in fins along their backs, and that species with unquestionable feathers, such as oviraptorosaurs and dromaeosaurs are not dinosaurs, but true birds unrelated to dinosaurs. However, a majority of studies have concluded that feathered dinosaurs are in fact dinosaurs, and that the simpler filaments of unquestionable theropods represent simple feathers. Some researchers have demonstrated the presence of color-bearing melanin in the structures—which would be expected in feathers but not collagen fibers. Others have demonstrated, using studies of modern bird decomposition, that even advanced feathers appear filamentous when subjected to the crushing forces experienced during fossilization, and that the supposed "protofeathers" may have been more complex than previously thought. Detailed examination of the "protofeathers" of Sinosauropteryx prima showed that individual feathers consisted of a central quill (rachis) with thinner barbs branching off from it, similar to but more primitive in structure than modern bird feathers.

The 2022 description of branched feathers in the pterosaur Tupandactylus provides strong evidence that "pycnofibers" are not actually a distinct integument unrelated to origin of feathers. The most parsimonious scenario is the presence of feathers in the last common ancestor of pterosaurs and dinosaurs already in the Early Triassic. Tupandactylus's melanosomes indicate visual signalling was an important factor in the evolution of feathers.

Fossil cast of NGMC 91, a probable specimen of Sinornithosaurus

Skeleton

Because feathers are often associated with birds, feathered dinosaurs are often touted as the "missing link" between birds and other dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important proof for paleontologists.

Comparisons of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called Maniraptora. Skeletal similarities include the skull, tooth build, neck, uncinate processes on the ribs, an open hip socket, a retroverted long pubis, flexible wrist (semi-lunate carpal), long arms, three-fingered hand, general pectoral girdle, shoulder blade, furcula, and breast bones. Almost all skeletal traits of Archaeopteryx can be found in non-avian maniraptorans.

A study comparing embryonic, juvenile and adult archosaur skulls concluded that bird skulls are derived from those of theropod dinosaurs by progenesis, a type of paedomorphic heterochrony, which resulted in retention of juvenile characteristics of their ancestors.

Lungs

Comparison between the air sacs of Majungasaurus and a bird (duck)

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation led by Patrick M. O'Connor of Ohio University. In theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) flexible soft tissue air sacs likely pumped air through the stiff lungs, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said.

Heart

Computed tomography (CT) scans conducted in 2000 of the chest cavity of a specimen of the ornithopod Thescelosaurus found the apparent remnants of a complex four-chambered heart, much like those found in today's mammals and birds. The idea is controversial within the scientific community, criticised for being bad anatomical science or simply wishful thinking, It is also not very surprising as crocodilians also possess four-chambered hearts.

A study published in 2011 applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. From these methods, the authors found that: the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich's heart; the "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar minerals, quartz, and gypsum, as well as some plant fragments; carbon, nitrogen, and phosphorus, chemical elements important to life, were lacking in their samples; and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.

The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity. Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.

Sleeping posture

Fossils of the troodonts Mei and Sinornithoides demonstrate that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.

Reproductive biology

When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed that the specimen is female. Further research has found medullary bone in the theropod Allosaurus and ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.

Brooding and care of young

A nesting Citipati osmolskae specimen, at the American Museum of Natural History in New York City

Several Citipati specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding.

Numerous dinosaur species, for example Maiasaura, have been found in herds mixing both very young and adult individuals, suggesting rich interactions between them.

A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth.

Gizzard stones

Both birds and dinosaurs use gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths. Gizzard stones are also found in some fish (mullets, mud shad, and the gillaroo, a type of trout) and in crocodiles.

Molecular evidence

On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. Several proteins have putatively been detected in dinosaur fossils, including hemoglobin. In 2023, beta-protein structures were reported from the feathers of the dinosaur Sinornithosaurus and the early bird Confuciusornis. This confirms that ancient feathers had a composition similar to that of modern birds. Some fossil feathers were reported to have a composition rich in alpha proteins, but fossilization experiments demonstrate that this protein composition is simply an artefact of preservation, because beta-sheet protein structures are readily transformed to alpha-helices during thermal maturation.

In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone of specimen MOR 1125 from the Hell Creek Formation in Montana. The seven collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken), suggest that older theropods and birds are closely related. The soft tissue allowed a molecular comparison of cellular anatomy and protein sequencing of collagen tissue published in 2007, both of which indicated that T. rex and birds are more closely related to each other than either is to Alligator. A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of the "mummified" Brachylophosaurus canadensis specimen MOR 2598, a hadrosaur. However, these results have been very controversial. No other peptides of a Mesozoic age have been reported. In 2008, it was suggested that the presumed soft tissue was in fact a bacterial microfilm. In response, it was argued that these very microfilms protected the soft tissue. Another objection was that the results could have been caused by contamination. In 2015, under more controlled conditions safeguarding against contamination, the peptides were still identified. In 2017, a study found that a peptide was present in the bone of the modern ostrich that was identical to that found in the Tyrannosaurus and Brachylophosaurus specimens, highlighting the danger of a cross-contamination.

The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed.

Origin of bird flight

Main article: Origin of avian flight

Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurs, which arose soon after the discovery of Archaeopteryx in 1862. Two theories have dominated most of the discussion since then: the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as trees, which would help small feathered dinosaurs escape from predators.

In March 2018, scientists reported that Archaeopteryx was likely capable of flight, but in a manner substantially different from that of modern birds.

Cursorial ("from the ground up") theory

Reconstruction of Rahonavis, a ground-dwelling feathered dinosaur that some researchers think was well equipped for flight

The cursorial theory of the origin of flight was first proposed by Samuel Wendell Williston, and elaborated upon by Baron Nopcsa. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.

Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators keep their balance, and that the scales of the forearms elongated, evolving into feathers. The feathers could also have been used to trap insects or other prey. Progressively, the animals leapt for longer distances, helped by their evolving wings. Nopcsa also proposed three stages in the evolution of flight. First, animals developed passive flight, in which developing wing structures served as a sort of parachute. Second, they achieved active flight by flapping the wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar.

Proposed development of flight in a book from 1922: Tetrapteryx, Archaeopteryx, Hypothetical Stage, Modern Bird

While some authors had rejected the homology between feathers and scales due to their different proteins, recent studies provide evidence that those structures do share a common origin.However, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries show that feathers evolved millions of years before flight.

Feathers are very common in coelurosaurian dinosaurs (including the early tyrannosauroid Dilong). Modern birds are classified as coelurosaurs by nearly all palaeontologists, though not by a few ornithologists. The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners in their hunting style) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.

All of the Archaeopteryx fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the Jesus Christ Lizard (common basilisk).

Most recent opposition to the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The criticism is based on embryological analyses that suggest birds' wings are formed from digits 2, 3, and 4, (corresponding to the index, middle, and ring fingers in humans. The first of a bird's three digits forms the alula, which they use to avoid stalling in low-speed flight—for example, when landing). The hands of coelurosaurs, however, are formed by digits 1, 2, and 3 (thumb and first two fingers in humans). However, these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in clades that have lost some digits in the course of their evolution, and that birds' "hands" do develop from digits 1, 2, and 3. For more information about this subject, see "Digit homology".

Fowler et al. (2011) proposed a model explaining how dromaeosaurids may have hunted. The animal would use its wing as stabilizers while standing on top of its prey eating it alive in the manner of an eagle or a hawk. The authors consider this an important addition to the topic of how flapping movements evolved, arguing they likely precede flight.

Wing-assisted incline running

The wing-assisted incline running (WAIR) hypothesis was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. This makes it a specialized type of cursorial ("from the ground up") theory. Note that in this scenario birds need downforce to give their feet increased grip. But early birds, including Archaeopteryx, lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes. Since the downforce WAIR depends on is generated by upstrokes, it seems that early birds were incapable of WAIR. Because WAIR is a behavioural trait without osteological specializations, the phylogenetic placement of the flight stroke before the divergence of the Neornithes, the group which contains all extant birds, makes it impossible to determine if WAIR is ancestral to the avian flight stroke or derived from it.

Arboreal ("from the trees down") theory

The four-winged Microraptor, a member of the Dromaeosauridae, a group of dinosaurs closely related to birds

Most versions of the arboreal hypothesis state that the ancestors of birds were very small dinosaurs that lived in trees, springing from branch to branch. This small dinosaur already had feathers, which were co-opted by evolution to produce longer, stiffer forms that were useful in aerodynamics, eventually producing wings. Wings would have then evolved and become increasingly refined as devices to give the leaper more control, to parachute, to glide, and to fly in stepwise fashion. The arboreal hypothesis also notes that, for arboreal animals, aerodynamics are far more energy efficient, since such animals simply fall to achieve minimum gliding speeds.

Several small dinosaurs from the Jurassic or Early Cretaceous, all with feathers, have been interpreted as possibly having arboreal and/or aerodynamic adaptations. These include Scansoriopteryx, Epidexipteryx, Microraptor, Pedopenna, and Anchiornis. Anchiornis is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before Archaeopteryx.

Analysis of the proportions of the toe bones of the most primitive birds Archaeopteryx and Confuciusornis, compared to those of living species, suggest that the early species may have lived both on the ground and in trees.

One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds.

Diminished significance of Archaeopteryx

Archaeopteryx was the first and for a long time the only known feathered Mesozoic animal. As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mid-1990s.

The supracoracoideus works using a pulley-like system to lift the wing while the pectorals provide the powerful downstroke.

There has been debate about whether Archaeopteryx could really fly. It appears that Archaeopteryx had the brain structures and inner-ear balance sensors that birds use to control their flight. Archaeopteryx also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes (see diagram above of supracoracoideus pulley); this may mean that it and other early birds were incapable of flapping flight and could only glide.

But the discovery since the early 1990s of many feathered dinosaurs means that Archaeopteryx is no longer the key figure in the evolution of bird flight. Other small feathered coelurosaurs from the Cretaceous and Late Jurassic show possible precursors of avian flight. These include Rahonavis, a ground-runner with a Velociraptor-like raised sickle claw on the second toe, that some paleontologists assume to have been better adapted for flight than Archaeopteryx, Scansoriopteryx, an arboreal dinosaur that may support the "from the trees down" theory, and Microraptor, an arboreal dinosaur possibly capable of powered flight but, if so, more like a biplane, as it had well-developed feathers on its legs. As early as 1915, some scientists argued that the evolution of bird flight may have gone through a four-winged (or tetrapteryx) stage. Hartman et al. (2019) found that, because of how basal flying paravians are phylogenetically distributed, flight most likely evolved five times among paravians instead of only once. Yi, Archaeopteryx, Rahonavis and Microraptor were thus considered examples of convergent evolution instead of precursors of bird flight.

Secondary flightlessness in dinosaurs

A minority hypothesis, credited to the books Predatory Dinosaurs of the World (1988) and Dinosaurs of the Air (2002) by scientific illustrator Gregory Paul, suggests that some groups of non-flying carnivorous dinosaurs — especially deinonychosaurs, but perhaps others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs — actually descend from birds or other flighted maniraptorans. Paul also proposed that the ancestors of these groups were more advanced in their flight adaptations than Archaeopteryx. The hypothesis would mean that Archaeopteryx is less closely related to extant birds than these dinosaurs are. In 2016, Paul suggested that omnivoropterygid avialans were closely related to oviraptorosaurs and that jeholornithid avialans were closely related to therizinosaurs; he considered them to not be avians but suggested that they shared a flighted ancestor.

Mayr et al. (2005) analyzed a new, tenth specimen of Archaeopteryx, and concluded that Archaeopteryx was the sister clade to the Deinonychosauria, but that the more advanced bird Confuciusornis was within the Dromaeosauridae. This paper, however, excluded all other birds and thus did not sample their character distributions. The paper was criticized by Corfe and Butler (2006) who found the authors could not support their conclusions statistically. Mayr et al. agreed that the statistical support for the authors' earlier paper was weak but stated that it is also weak for the alternative scenarios.

Most subsequent cladistic analyses, an exception being that of Hartman and colleagues (2019), do not support Paul's hypothesis about the position of Archaeopteryx. Instead, they indicate that Archaeopteryx is closer to birds, within the clade Avialae, than it is to deinonychosaurs or oviraptorosaurs. Microraptor, Pedopenna, and Anchiornis all have winged feet, share many features, and lie close to the base of the clade Paraves. This suggests that the ancestral paravian may have been a four-winged glider. Deinonychus may also display partial volancy, with the young being capable of flight or gliding and the adults being flightless. In 2018, a study concluded that the last common ancestor of the Pennaraptora had joint surfaces on the fingers, and between the metatarsus and the wrist, that were optimised to stabilise the hand in flight. This was seen as an indication for secondary flightlessness in heavy basal members of that group.

In Euornithes, the earliest unequivocal example of secondary flightlessness is Patagopteryx.