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Friday, December 5, 2025

Progress

From Wikipedia, the free encyclopedia

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

Progress is movement towards a perceived refined, improved, or otherwise desired state. It is central to the philosophy of progressivism, which interprets progress as the set of advancements in technology, science, and social organization efficiency – the latter being generally achieved through direct societal action, as in social enterprise or through activism, but being also attainable through natural sociocultural evolution – that progressivism holds all human societies should strive towards.

The concept of progress was introduced in the early-19th-century social theories, especially social evolution as described by Auguste Comte and Herbert Spencer. It was present in the Enlightenment's philosophies of history. As a goal, social progress has been advocated by varying realms of political ideologies with different theories on how it is to be achieved.

Measuring progress

Specific indicators for measuring progress can range from economic data, technical innovations, change in the political or legal system, and questions bearing on individual life chances, such as life expectancy and risk of disease and disability.

GDP growth has become a key orientation for politics and is often taken as a key figure to evaluate a politician's performance. However, GDP has a number of flaws that make it a bad measure of progress, especially for developed countries. For example, environmental damage is not taken into account nor is the sustainability of economic activity. Wikiprogress has been set up to share information on evaluating societal progress. It aims to facilitate the exchange of ideas, initiatives and knowledge. HumanProgress.org is another online resource that seeks to compile data on different measures of societal progress.

Our World in Data is a scientific online publication, based at the University of Oxford, that studies how to make progress against large global problems such as poverty, disease, hunger, climate change, war, existential risks, and inequality. The mission of Our World in Data is to present "research and data to make progress against the world’s largest problems".

The Social Progress Index is a tool developed by the International Organization Imperative Social Progress, which measures the extent to which countries cover social and environmental needs of its citizenry. There are fifty-two indicators in three areas or dimensions: Basic Human Needs, and Foundations of Wellbeing and Opportunities which show the relative performance of nations.

Indices that can be used to measure progress include:

Scientific progress

Scientific progress is the idea that the scientific community learns more over time, which causes a body of scientific knowledge to accumulate. The chemists in the 19th century knew less about chemistry than the chemists in the 20th century, and they in turn knew less than the chemists in the 21st century. Looking forward, today's chemists reasonably expect that chemists in future centuries will know more than they do.

From the 18th century through late 20th century, the history of science, especially of the physical and biological sciences, was often presented as a progressive accumulation of knowledge, in which true theories replaced false beliefs. Some more recent historical interpretations, such as those of Thomas Kuhn, tend to portray the history of science in terms of competing paradigms or conceptual systems in a wider matrix of intellectual, cultural, economic and political trends. These interpretations, however, have met with opposition for they also portray the history of science as an incoherent system of incommensurable paradigms, not leading to any scientific progress, but only to the illusion of progress.

Whether other intellectual disciplines make progress in the same way as the sciences is a matter of debate. For example, one might expect that today's historians know more about global history than their ancient counterparts (consider the histories of Herodotus). Yet, knowledge can be lost through the passage of time, or the criteria for evaluating what is worth knowing can change. Similarly, there is considerable disagreement over whether fields such as philosophy make progress - or even whether they aim at accumulating knowledge in the same way as the sciences.

Social progress

Aspects of social progress, as described by Condorcet, have included the disappearance of slavery, the rise of literacy, the lessening of inequalities between the sexes, reforms of harsh prisons and the decline of poverty. The social progress of a society can be measured based on factors such as its ability to address fundamental human needs, help citizens improve their quality of life, and provide opportunities for citizens to succeed.

Social progress is often improved by increases in GDP, although other factors are also relevant. An imbalance between economic and social progress hinders further economic progress, and can lead to political instability. Where there is an imbalance between economic growth and social progress, political instability and unrest often arise. Lagging social progress also holds back economic growth in these and other countries that fail to address human needs, build social capital, and create opportunity for their citizens.

Status of women

How progress improved the status of women in traditional society was a major theme of historians starting in the Enlightenment and continuing to today. British theorists William Robertson (1721–1793) and Edmund Burke (1729–1797), along with many of their contemporaries, remained committed to Christian- and republican-based conceptions of virtue, while working within a new Enlightenment paradigm. The political agenda related beauty, taste, and morality to the imperatives and needs of modern societies of a high level of sophistication and differentiation. Two themes in the work of Robertson and Burke—the nature of women in 'savage' and 'civilized' societies and 'beauty in distress'—reveals how long-held convictions about the character of women, especially with regard to their capacity and right to appear in the public domain, were modified and adjusted to the idea of progress and became central to modern European civilization.

Classics experts have examined the status of women in the ancient world, concluding that in the Roman Empire, with its superior social organization, internal peace, and rule of law, allowed women to enjoy a somewhat better standing than in ancient Greece, where women were distinctly inferior. The inferior status of women in traditional China has raised the issue of whether the idea of progress requires a thoroughgoing rejection of traditionalism—a belief held by many Chinese reformers in the early 20th century.

Historians Leo Marx and Bruce Mazlish asking, "should we in fact abandon the idea of progress as a view of the past," answer that there is no doubt "that the status of women has improved markedly" in cultures that have adopted the Enlightenment idea of progress.

Modernization

Modernization was promoted by classical liberals in the 19th and 20th centuries, who called for the rapid modernization of the economy and society to remove the traditional hindrances to free markets and free movements of people. During the Enlightenment in Europe social commentators and philosophers began to realize that people themselves could change society and change their way of life. Instead of being made completely by gods, there was increasing room for the idea that people themselves made their own society—and not only that, as Giambattista Vico argued, because people made their own society, they could also fully comprehend it. This gave rise to new sciences, or proto-sciences, which claimed to provide new scientific knowledge about what society was like, and how one may change it for the better.

In turn, this gave rise to progressive opinion, in contrast with conservational opinion. The social conservationists were skeptical about panaceas for social ills. According to conservatives, attempts to radically remake society normally make things worse. Edmund Burke was the leading exponent of this, although later-day liberals like Friedrich Hayek have espoused similar views. They argue that society changes organically and naturally, and that grand plans for the remaking of society, like the French Revolution, National Socialism and Communism hurt society by removing the traditional constraints on the exercise of power.

The scientific advances of the 16th and 17th centuries provided a basis for Francis Bacon's book the New Atlantis. In the 17th century, Bernard le Bovier de Fontenelle described progress with respect to arts and the sciences, saying that each age has the advantage of not having to rediscover what was accomplished in preceding ages. The epistemology of John Locke provided further support and was popularized by the Encyclopedists Diderot, Holbach, and Condorcet. Locke had a powerful influence on the American Founding Fathers.^[1] The first complete statement of progress is that of Turgot, in his "A Philosophical Review of the Successive Advances of the Human Mind" (1750). For Turgot, progress covers not only the arts and sciences but, on their base, the whole of culture—manner, mores, institutions, legal codes, economy, and society. Condorcet predicted the disappearance of slavery, the rise of literacy, the lessening of inequalities between the sexes, reforms of harsh prisons and the decline of poverty.

John Stuart Mill's (1806–1873) ethical and political thought demonstrated faith in the power of ideas and of intellectual education for improving human nature or behavior. For those who do not share this faith the idea of progress becomes questionable.

Alfred Marshall (1842–1924), a British economist of the early 20th century, was a proponent of classical liberalism. In his highly influential Principles of Economics (1890), he was deeply interested in human progress and in what is now called sustainable development. For Marshall, the importance of wealth lay in its ability to promote the physical, mental, and moral health of the general population. After World War II, the modernization and development programs undertaken in the Third World were typically based on the idea of progress.

In Russia the notion of progress was first imported from the West by Peter the Great (1672–1725). An absolute ruler, he used the concept to modernize Russia and to legitimize his monarchy (unlike its usage in Western Europe, where it was primarily associated with political opposition). By the early 19th century, the notion of progress was being taken up by Russian intellectuals and was no longer accepted as legitimate by the tsars. Four schools of thought on progress emerged in 19th-century Russia: conservative (reactionary), religious, liberal, and socialist—the latter winning out in the form of Bolshevist materialism.

The intellectual leaders of the American Revolution, such as Benjamin Franklin, Thomas Paine, Thomas Jefferson and John Adams, were immersed in Enlightenment thought and believed the idea of progress meant that they could reorganize the political system to the benefit of the human condition; both for Americans and also, as Jefferson put it, for an "Empire of Liberty" that would benefit all mankind. In particular, Adams wrote “I must study politics and war, that our sons may have liberty to study mathematics and philosophy. Our sons ought to study mathematics and philosophy, geography, natural history and naval architecture, navigation, commerce and agriculture in order to give their children a right to study painting, poetry, music, architecture, statuary, tapestry and porcelain.”

Juan Bautista Alberdi (1810–1884) was one of the most influential political theorists in Argentina. Economic liberalism was the key to his idea of progress. He promoted faith in progress, while chiding fellow Latin Americans for blind copying of United States and Europe models. He hoped for progress through promotion of immigration, education, and a moderate type of federalism and republicanism that might serve as a transition in Argentina to true democracy.

In Mexico, José María Luis Mora (1794–1850) was a leader of classical liberalism in the first generation after independence, leading the battle against the conservative trinity of the army, the church, and the hacendados. He envisioned progress as both a process of human development by the search for philosophical truth and as the introduction of an era of material prosperity by technological advancement. His plan for Mexican reform demanded a republican government bolstered by widespread popular education free of clerical control, confiscation and sale of ecclesiastical lands as a means of redistributing income and clearing government debts, and effective control of a reduced military force by the government. Mora also demanded the establishment of legal equality between native Mexicans and foreign residents. His program, untried in his lifetime, became the key element in the Mexican Constitution of 1857.

In Italy, the idea that progress in science and technology would lead to solutions for human ills was connected to the nationalism that united the country in 1860. The Piedmontese Prime Minister Camillo Cavour envisaged the railways as a major factor in the modernization and unification of the Italian peninsula. The new Kingdom of Italy, formed in 1861, worked to speed up the processes of modernization and industrialization that had begun in the north, but were slow to arrive in the Papal States and central Italy, and were nowhere in sight in the "Mezzogiorno" (that is, Southern Italy and Sicily). The government sought to combat the backwardness of the poorer regions in the south and work towards augmenting the size and quality of the newly created Italian army so that it could compete on an equal footing with the powerful nations of Europe. In the same period, the government was legislating in favour of public education to fight the great problem of illiteracy, upgrade the teaching classes, improve existing schools, and procure the funds needed for social hygiene and care of the body as factors in the physical and moral regeneration of the race.

In China, in the 20th century the Kuomintang or Nationalist party, which ruled from the 1920s to the 1940s, advocated progress. The Communists under Mao Zedong adopted different models and their ruinous projects caused mass famines. After Mao's death, however, the new regime led by Deng Xiaoping (1904–1997) and his successors aggressively promoted modernization of the economy using capitalist models and imported western technology. This was termed the "Opening of China" in the West, and more broadly encompasses Chinese economic reform.

Among environmentalists, there is a continuum between two opposing poles. The one pole is optimistic, progressive, and business-oriented, and endorses the classic idea of progress. For example, bright green environmentalism endorses the idea that new designs, social innovations and green technologies can solve critical environmental challenges. The other is pessimistic in respect of technological solutions, warning of impending global crisis (through climate change or peak oil, for example) and tends to reject the very idea of modernity and the myth of progress that is so central to modernization thinking. Similarly, Kirkpatrick Sale, wrote about progress as a myth benefiting the few, and a pending environmental doomsday for everyone. An example is the philosophy of Deep Ecology.

Philosophy

Sociologist Robert Nisbet said that "No single idea has been more important than ... the Idea of Progress in Western civilization for three thousand years", and defines five "crucial premises" of the idea of progress:

value of the past
nobility of Western civilization
worth of economic/technological growth
faith in reason and scientific/scholarly knowledge obtained through reason
intrinsic importance and worth of life on earth

Sociologist P. A. Sorokin said, "The ancient Chinese, Babylonian, Hindu, Greek, Roman, and most of the medieval thinkers supporting theories of rhythmical, cyclical or trendless movements of social processes were much nearer to reality than the present proponents of the linear view." Unlike Confucianism and to a certain extent Taoism, that both search for an ideal past, the Judeo-Christian-Islamic tradition believes in the fulfillment of history, which was translated into the idea of progress in the modern age. Therefore, Chinese proponents of modernization have looked to western models. According to Thompson, the late Qing dynasty reformer, Kang Youwei, believed he had found a model for reform and "modernisation" in the Ancient Chinese Classics.

Philosopher Karl Popper said that progress was not fully adequate as a scientific explanation of social phenomena. More recently, Kirkpatrick Sale, a self-proclaimed neo-luddite author, wrote exclusively about progress as a myth, in an essay entitled "Five Facets of a Myth".

Iggers (1965) says that proponents of progress underestimated the extent of man's destructiveness and irrationality, while critics misunderstand the role of rationality and morality in human behavior.

In 1946, psychoanalyst Charles Baudouin claimed modernity has retained the "corollary" of the progress myth, the idea that the present is superior to the past, while at the same time insisting that it is free of the myth:

The last two centuries were familiar with the myth of progress. Our own century has adopted the myth of modernity. The one myth has replaced the other.
Men ceased to believe in progress; but only to pin their faith to more tangible realities, whose sole original significance had been that they were the instruments of progress.
This exaltation of the present ... is a corollary of that very faith in progress which people claim to have discarded. The present is superior to the past, by definition, only in a mythology of progress. Thus one retains the corollary while rejecting the principle. There is only one way of retaining a position of whose instability one is conscious. One must simply refrain from thinking.

A cyclical theory of history was adopted by Oswald Spengler (1880–1936), a German historian who wrote The Decline of the West in 1920. World War I, World War II, and the rise of totalitarianism demonstrated that progress was not automatic and that technological improvement did not necessarily guarantee democracy and moral advancement. British historian Arnold J. Toynbee (1889–1975) felt that Christianity would help modern civilization overcome its challenges.

The Jeffersonians said that history is not exhausted but that man may begin again in a new world. Besides rejecting the lessons of the past, they Americanized the idea of progress by democratizing and vulgarizing it to include the welfare of the common man as a form of republicanism. As Romantics deeply concerned with the past, collecting source materials and founding historical societies, the Founding Fathers were animated by clear principles. They saw man in control of his destiny, saw virtue as a distinguishing characteristic of a republic, and were concerned with happiness, progress, and prosperity. Thomas Paine, combining the spirit of rationalism and romanticism, pictured a time when America's innocence would sound like a romance, and concluded that the fall of America could mark the end of "the noblest work of human wisdom".

Historian J. B. Bury wrote in 1920: the desirable outcome of human development would be a condition of society in which all the inhabitants of the planet would enjoy a perfectly happy existence. ... It cannot be proved that the unknown destination towards which man is advancing is desirable. The movement may be Progress, or it may be in an undesirable direction and therefore not Progress. ... The Progress of humanity belongs to the same order of ideas as Providence or personal immortality. It is true or it is false, and like them it cannot be proved either true or false. Belief in it is an act of faith.

In the postmodernist thought steadily gaining ground from the 1980s, the grandiose claims of the modernizers are steadily eroded, and the very concept of social progress is again questioned and scrutinized. In the new vision, radical modernizers like Joseph Stalin and Mao Zedong appear as totalitarian despots, whose vision of social progress is held to be totally deformed. Postmodernists question the validity of 19th-century and 20th-century notions of progress—both on the capitalist and the Marxist side of the spectrum. They argue that both capitalism and Marxism overemphasize technological achievements and material prosperity while ignoring the value of inner happiness and peace of mind. Postmodernism posits that both dystopia and utopia are one and the same, overarching grand narratives with impossible conclusions.

Some 20th-century authors refer to the "Myth of Progress" to refer to the idea that the human condition will inevitably improve. In 1932, English physician Montague David Eder wrote: "The myth of progress states that civilization has moved, is moving, and will move in a desirable direction. Progress is inevitable... Philosophers, men of science and politicians have accepted the idea of the inevitability of progress." Eder argues that the advancement of civilization is leading to greater unhappiness and loss of control in the environment. The strongest critics of the idea of progress complain that it remains a dominant idea in the 21st century, and shows no sign of diminished influence. As one fierce critic, British historian John Gray (b. 1948), concludes:^[42]

Faith in the liberating power of knowledge is encrypted into modern life. Drawing on some of Europe's most ancient traditions, and daily reinforced by the quickening advance of science, it cannot be given up by an act of will. The interaction of quickening scientific advance with unchanging human needs is a fate that we may perhaps temper, but cannot overcome... Those who hold to the possibility of progress need not fear. The illusion that through science humans can remake the world is an integral part of the modern condition. Renewing the eschatological hopes of the past, progress is an illusion with a future.

Recently the idea of progress has been generalized to psychology, being related with the concept of a goal, that is, progress is understood as "what counts as a means of advancing towards the end result of a given defined goal."

Antiquity

Historian J. B. Bury said that thought in ancient Greece was dominated by the theory of world-cycles or the doctrine of eternal return, and was steeped in a belief parallel to the Judaic "fall of man," but rather from a preceding "Golden Age" of innocence and simplicity. Time was generally regarded as the enemy of humanity which depreciates the value of the world. He credits the Epicureans with having had a potential for leading to the foundation of a theory of progress through their materialistic acceptance of the atomism of Democritus as the explanation for a world without an intervening deity.

For them, the earliest condition of men resembled that of the beasts, and from this primitive and miserable condition they laboriously reached the existing state of civilisation, not by external guidance or as a consequence of some initial design, but simply by the exercise of human intelligence throughout a long period.

Robert Nisbet and Gertrude Himmelfarb have attributed a notion of progress to other Greeks. Xenophanes said "The gods did not reveal to men all things in the beginning, but men through their own search find in the course of time that which is better."

Islamic era

With the rise of the Umayyad and Abbasid caliphates and later Ottoman Empire, progress in the Islamic civilizations was characterized by a system of translating books (particularly Greek philosophy books in the Abbasid era) of various cultures into local languages (often Arabic and Persian), testing and refining their scientific or philosophical theories and claims, and then building upon them with their own Islamic ideas, theologies, ontologies, and scientific experimental results. The Round city of Baghdad was characterized as a model and example of progress for the region, where peoples of every religion and race sent their top students to study at its famous international academy called the House of Wisdom. Islamic Spain was also famed as a center of learning in Europe, where Jews and Christians flocked to Muslim halaqas, eager to bring the latest knowledge back to their countries in Europe, which later sparked the European Renaissance due the Muslim scholars' finesse in adapting classical knowledge (such as Greek philosophy) to Abrahamic contexts. Muslim rulers viewed knowledge, including both scientific and philosophical knowledge, as a key to power, and promoted learning, scientific inquiry, and patronization of scholars.

Renaissance

During the Medieval period, science was to a large extent based on Scholastic (a method of thinking and learning from the Middle Ages) interpretations of Aristotle's work. The Renaissance changed the mindset in Europe, which induced a revolution in curiosity about nature in general and scientific advance, which opened the gates for technical and economic advance. Furthermore, the individual potential was seen as a never-ending quest for being God-like, paving the way for a view of man based on unlimited perfection and progress.

Age of Enlightenment (1650–1800)

In the Enlightenment, French historian and philosopher Voltaire (1694–1778) was a major proponent of progress. At first Voltaire's thought was informed by the idea of progress coupled with rationalism. His subsequent notion of the historical idea of progress saw science and reason as the driving forces behind societal advancement.

Immanuel Kant (1724–1804) argued that progress is neither automatic nor continuous and does not measure knowledge or wealth, but is a painful and largely inadvertent passage from barbarism through civilization toward enlightened culture and the abolition of war. Kant called for education, with the education of humankind seen as a slow process whereby world history propels mankind toward peace through war, international commerce, and enlightened self-interest.

Scottish theorist Adam Ferguson (1723–1816) defined human progress as the working out of a divine plan, though he rejected predestination. The difficulties and dangers of life provided the necessary stimuli for human development, while the uniquely human ability to evaluate led to ambition and the conscious striving for excellence. But he never adequately analyzed the competitive and aggressive consequences stemming from his emphasis on ambition even though he envisioned man's lot as a perpetual striving with no earthly culmination. Man found his happiness only in effort.

Some scholars consider the idea of progress that was affirmed with the Enlightenment, as a secularization of ideas from early Christianity, and a reworking of ideas from ancient Greece.

Romanticism and 19th century

In the 19th century, Romantic critics charged that progress did not automatically better the human condition, and in some ways could make it worse. Thomas Malthus (1766–1834) reacted against the concept of progress as set forth by William Godwin and Condorcet because he believed that inequality of conditions is "the best (state) calculated to develop the energies and faculties of man". He said, "Had population and food increased in the same ratio, it is probable that man might never have emerged from the savage state." He argued that man's capacity for improvement has been demonstrated by the growth of his intellect, a form of progress which offsets the distresses engendered by the law of population.

German philosopher Friedrich Nietzsche (1844–1900) criticized the idea of progress as the 'weakling's doctrines of optimism,' and advocated undermining concepts such as faith in progress, to allow the strong individual to stand above the plebeian masses. An important part of his thinking consists of the attempt to use the classical model of 'eternal recurrence of the same' to dislodge the idea of progress.

Iggers (1965) argues there was general agreement in the late 19th century that the steady accumulation of knowledge and the progressive replacement of conjectural, that is, theological or metaphysical, notions by scientific ones was what created progress. Most scholars concluded this growth of scientific knowledge and methods led to the growth of industry and the transformation of warlike societies into industrial and pacific ones. They agreed as well that there had been a systematic decline of coercion in government, and an increasing role of liberty and of rule by consent. There was more emphasis on impersonal social and historical forces; progress was increasingly seen as the result of an inner logic of society.

Marxist theory (late 19th century)

Marx developed a theory of historical materialism. He describes the mid-19th-century condition in The Communist Manifesto as follows:

The bourgeoisie cannot exist without constantly revolutionizing the instruments of production, and thereby the relations of production, and with them the whole relations of society. Conservation of the old modes of production in unaltered form, was, on the contrary, the first condition of existence for all earlier industrial classes. Constant revolutionizing of production, uninterrupted disturbance of all social conditions, everlasting uncertainty, and agitation distinguish the bourgeois epoch from all earlier ones. All fixed, fast frozen relations, with their train of ancient and venerable prejudices and opinions, are swept away, all new-formed ones become antiquated before they can ossify. All that is solid melts into air, all which is holy is profaned, and man is at last compelled to face with sober senses his real condition of life and his relations with his kind.

Furthermore, Marx described the process of social progress, which in his opinion is based on the interaction between the productive forces and the relations of production:

No social order is ever destroyed before all the productive forces for which it is sufficient have been developed, and new superior relations of production never replace older ones before the material conditions for their existence have matured within the framework of the old society.

Capitalism is thought by Marx as a process of continual change, in which the growth of markets dissolve all fixities in human life, and Marx argues that capitalism is progressive and non-reactionary. Marxism further states that capitalism, in its quest for higher profits and new markets, will inevitably sow the seeds of its own destruction. Marxists believe that, in the future, capitalism will be replaced by socialism and eventually communism.

Many advocates of capitalism such as Schumpeter agreed with Marx's analysis of capitalism as a process of continual change through creative destruction, but, unlike Marx, believed and hoped that capitalism could essentially go on forever.

Thus, by the beginning of the 20th century, two opposing schools of thought—Marxism and liberalism—believed in the possibility and the desirability of continual change and improvement. Marxists strongly opposed capitalism and the liberals strongly supported it, but the one concept they could both agree on was progress, which affirms the power of human beings to make, improve and reshape their society, with the aid of scientific knowledge, technology and practical experimentation. Modernity denotes cultures that embrace that concept of progress. (This is not the same as modernism, which was the artistic and philosophical response to modernity, some of which embraced technology while rejecting individualism, but more of which rejected modernity entirely.)

Evolution of the brain

From Wikipedia, the free encyclopedia

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

The evolution of the brain refers to the progressive development and complexity of neural structures over millions of years, resulting in the diverse range of brain sizes and functions observed across different species today, particularly in vertebrates.

The evolution of the brain has exhibited diverging adaptations within taxonomic classes, such as Mammalia, and even more diverse adaptations across other taxonomic classes. Brain-to-body size scales allometrically. This means that as body size changes, so do other physiological, anatomical, and biochemical connections between the brain and body. Small-bodied mammals tend to have relatively large brains compared to their bodies, while larger mammals (such as whales) have smaller brain-to-body ratios. When brain weight is plotted against body weight for primates, the regression line of the sample points can indicate the brain power of a species. For example, lemurs fall below this line, suggesting that for a primate of their size, a larger brain would be expected. In contrast, humans lie well above this line, indicating they are more encephalized than lemurs and, in fact, more encephalized than any other primate. This suggests that human brains have undergone a larger evolutionary increase in complexity relative to size. Some of these changes have been linked to multiple genetic factors, including proteins and other organelles.

Early history

Unsolved problem in biology

How and why did the brain evolve?

Role of embryology

In addition to studying the fossil record, evolutionary history can be investigated via embryology. An embryo is an unborn/unhatched animal and evolutionary history can be studied by observing how processes in embryonic development are conserved (or not conserved) across species. Similarities between different species may indicate evolutionary connection. One way anthropologists study evolutionary connection between species is by observing orthologs. An ortholog is defined as two or more homologous genes between species that are evolutionarily related by linear descent. By using embryology the evolution of the brain can be tracked between various species.

Bone morphogenetic protein (BMP), a growth factor that plays a significant role in embryonic neural development, is highly conserved amongst vertebrates, as is sonic hedgehog (SHH), a morphogen that inhibits BMP to allow neural crest development. Tracking these growth factors with the use of embryology provides a deeper understanding of what areas of the brain diverged in their evolution. Varying levels of these growth factors lead to differing embryonic neural development which then in turn affects the complexity of future neural systems. Studying the brain's development at various embryonic stages across differing species provides additional insight into what evolutionary changes may have historically occurred. This then allows scientists to look into what factors may have caused such changes, such as links to neural network diversity, growth factor production, protein- coding selections, and other genetic factors.

Randomizing access and increasing size

Some animal phyla have gone through major brain enlargement through evolution (e.g. vertebrates and cephalopods both contain many lineages in which brains have grown through evolution) but most animal groups are composed only of species with extremely small brains. Some scientists argue that this difference is due to vertebrate and cephalopod neurons having evolved ways of communicating that overcome the scalability problem of neural networks while most animal groups have not. They argue that traditional neural networks fail to improve their function when scaled up because filtering based on previously known probabilities creates self-fulfilling prophecy-like biases. These biases generate false statistical evidence, producing a completely inaccurate worldview. In contrast, randomized access can overcome this problem, allowing brains to scale to more discriminating conditioned reflexes. This, in turn, can lead to new worldview-forming abilities once certain thresholds are reached. This means when neurons scale in a non randomized fashion that their functionality becomes more limited due to their neural networks being unable to process more complex systems without the exposure to new formations. This is explained by randomization allowing the entire brain to eventually get access to all information over the course of many shifts even though instant privileged access is physically impossible. They cite that vertebrate neurons transmit virus-like capsules containing RNA that are sometimes read in the neuron to which it is transmitted and sometimes passed further on unread which creates randomized access, and that cephalopod neurons make different proteins from the same gene which suggests another mechanism for randomization of concentrated information in neurons, both making it evolutionarily worth scaling up brains.

Brain re-organization

With the use of in vivo Magnetic resonance imaging (MRI) and tissue sampling, different cortical samples from members of each hominoid species were analyzed. In each species, specific areas were either relatively enlarged or shrunken, which can detail neural organizations. Different sizes in the cortical areas can show specific adaptations, functional specializations and evolutionary events that were changes in how the hominoid brain is organized. In early prediction it was thought that the frontal lobe, a large part of the brain that is generally devoted to behavior and social interaction, predicted the differences in behavior between hominoid and humans. Discrediting this theory was evidence supporting that damage to the frontal lobe in both humans and hominoids show atypical social and emotional behavior; thus, this similarity means that the frontal lobe was not very likely to be selected for reorganization. Instead, it is now believed that evolution occurred in other parts of the brain that are strictly associated with certain behaviors. The reorganization that took place is thought to have been more organizational than volumetric; whereas the brain volumes were relatively the same but specific landmark position of surface anatomical features, for example, the lunate sulcus suggest that the brains had been through a neurological reorganization. There is also evidence that the early hominin lineage also underwent a quiescent period, or a period of dormancy, which supports the idea of neural reorganization.

Dental fossil records for early humans and hominins show that immature hominins, including australopithecines and members of Homo, have a quiescent period (Bown et al. 1987). A quiescent period is a period in which there are no dental eruptions of adult teeth; at this time the child becomes more accustomed to social structure, and development of culture. During this time the child is given an extra advantage over other hominoids, devoting several years into developing speech and learning to cooperate within a community. This period is also discussed in relation to encephalization. It was discovered that chimpanzees do not have this neutral dental period, which suggests that a quiescent period occurred in very early hominin evolution. Using the models for neurological reorganization it can be suggested the cause for this period, dubbed middle childhood, is most likely for enhanced foraging abilities in varying seasonal environments.

Genetic factors in recent evolution

Genes involved in the neuro-development and in neuron physiology are extremely conserved between mammalian species (94% of genes expressed in common between humans and chimpanzees, 75% between humans and mice), compared to other organs. Therefore, few genes account for species differences in the human brain development and function.

Development of the human cerebral cortex

Main differences rely on the evolution of non-coding genomic regions, involved in the regulation of gene expression. This leads to differential expression of genes during the development of the human brain compared to other species, including chimpanzees. Some of these regions evolved fast in the human genome (human accelerated regions). The new genes expressed during human neurogenesis are notably associated with the NOTCH, WNT and mTOR pathways, but are also involved ZEB2, PDGFD and its receptor PDGFRβ. The human cerebral cortex is also characterized by a higher gradient of retinoic acid in the prefrontal cortex, leading to higher prefrontal cortex volume. All these differential gene expression lead to higher proliferation of the neural progenitors leading to more neurons in the human cerebral cortex. Some genes are lost in their expression during the development of the human cerebral cortex like GADD45G and FLRT2/FLRT3.

Another source of molecular novelty rely on new genes in the human or hominid genomes through segmental duplication. Around 30 new genes in the hominid genomes are dynamically expressed during human corticogenesis. Some were linked to higher proliferation of neural progenitors: NOTCH2NLA/B/C, ARHGAP11B, CROCCP2, TBC1D3, TMEM14B. Patients with deletions with NOTCH2NL genes display microcephaly, showing the necessity of such duplicated genes, acquired in the human genomes, in the proper corticogenesis.

MCPH1 and ASPM

Bruce Lahn, the senior author at the Howard Hughes Medical Center at the University of Chicago and colleagues have suggested that there are specific genes that control the size of the human brain. These genes continue to play a role in brain evolution, implying that the brain is continuing to evolve. The study began with the researchers assessing 214 genes that are involved in brain development. These genes were obtained from humans, macaques, rats and mice. Lahn and the other researchers noted points in the DNA sequences that caused protein alterations. These DNA changes were then scaled to the evolutionary time that it took for those changes to occur. The data showed the genes in the human brain evolved much faster than those of the other species. Once this genomic evidence was acquired, Lahn and his team decided to find the specific gene or genes that allowed for or even controlled this rapid evolution. Two genes were found to control the size of the human brain as it develops. These genes are Microcephalin (MCPH1) and Abnormal Spindle-like Microcephaly (ASPM). The researchers at the University of Chicago were able to determine that under the pressures of selection, both of these genes showed significant DNA sequence changes. Lahn's earlier studies displayed that Microcephalin experienced rapid evolution along the primate lineage which eventually led to the emergence of Homo sapiens. After the emergence of humans, Microcephalin seems to have shown a slower evolution rate. On the contrary, ASPM showed its most rapid evolution in the later years of human evolution once the divergence between chimpanzees and humans had already occurred.

Each of the gene sequences went through specific changes that led to the evolution of humans from ancestral relatives. In order to determine these alterations, Lahn and his colleagues used DNA sequences from multiple primates then compared and contrasted the sequences with those of humans. Following this step, the researchers statistically analyzed the key differences between the primate and human DNA to come to the conclusion, that the differences were due to natural selection. The changes in DNA sequences of these genes accumulated to bring about a competitive advantage and higher fitness that humans possess in relation to other primates. This comparative advantage is coupled with a larger brain size which ultimately allows the human mind to have a higher cognitive awareness.

ZEB2

ZEB2 is a protein- coding gene in the Homo sapien species. A 2021 study found that a delayed change in the shape of early brain cells causes the distinctly large human forebrain compared to other apes and identify ZEB2 as a genetic regulator of it, whose manipulation lead to acquisition of nonhuman ape cortical architecture in brain organoids.

NOVA1

In 2021, researchers reported that brain organoids created with stem cells into which they reintroduced the archaic gene variant NOVA1 present in Neanderthals and Denisovans via CRISPR-Cas9 shows that it has a major impact on neurodevelopment and that such genetic mutations during the evolution of the human brain underlie traits that separate modern humans from extinct Homo species. They found that expression of the archaic NOVA1 in cortical organoids leads to "modified synaptic protein interactions, affects glutamatergic signaling, underlies differences in neuronal connectivity, and promotes higher heterogeneity of neurons regarding their electrophysiological profiles". This research suggests positive selection of the modern NOVA1 gene, which may have promoted the randomization of neural scaling. A subsequent study failed to replicate the differences in organoid morphology between the modern human and the archaic NOVA1 variant, consistent with suspected unwanted side effects of CRISPR editing in the original study.

SRGAP2C and neuronal maturation

Less is known about neuronal maturation. Synaptic gene and protein expression are protracted, in line with the protracted synaptic maturation of human cortical neurons so called neoteny. This probably relies on the evolution of non-coding genomic regions. The consequence of the neoteny could be an extension of the period of synaptic plasticity and therefore of learning. A human-specific duplicated gene, SRGAP2C accounts for this synaptic neoteny and acts by regulating molecular pathways linked to neurodevelopmental disorders. Other genes are deferentially expressed in human neurons during their development such as osteocrin or cerebelin-2 .

LRRC37B and neuronal electrical properties

Even less is known about molecular specificities linked to the physiology of the human neurons. Human neurons are more divergent in the genes they express compared to chimpanzees than chimpanzees to gorilla, which suggests an acceleration of non-coding genomic regions associated with genes involved in neuronal physiology, in particular linked to the synapses. A hominid-specific duplicated gene, LRRC37B, codes for a transmembrane receptor that is selectively localized at the axon initial segment of human cortical pyramidal neurons. It inhibits their voltage-gated sodium channels that generate the action potentials leading to a lower neuronal excitability. Human cortical pyramidal neurons display a lower excitability compared to other mammalian species (including macaques and marmosets) which could lead to different circuit functions in the human species. Therefore, LRRC37B whose expression has been acquired in the human lineage after the separation from the chimpanzees could be a key gene in the function of the human cerebral cortex. LRRC37B binds to secreted FGF13A and SCN1B and modulate indirectly the activity of SCN8A, all involved in neural disorders such as epilepsy and autism. Therefore, LRRC37B may contribute to human-specific sensitivities to such disorders, both involved defects in neuronal excitability.

Genome repair

The genomic DNA of postmitotic neurons ordinarily does not replicate. Protection strategies have evolved to ensure the distinctive longevity of the neuronal genome. Human neurons are reliant on DNA repair processes to maintain function during an individual's life-time. DNA repair tends to occur preferentially at evolutionarily conserved sites that are specifically involved with the regulation of expression of genes essential for neuronal identity and function.

Other factors

Many other genetics may also be involved in recent evolution of the brain.

For instance, scientists showed experimentally, with brain organoids grown from stem cells, how differences between humans and chimpanzees are also substantially caused by non-coding DNA (often discarded as relatively meaningless "junk DNA") – in particular via CRE-regulated expression of the ZNF558 gene for a transcription factor that regulates the SPATA18 gene.^[31]^[32] SPATA18 gene encodes a protein and is able to influence lysosome-like organelles that are found within mitochondria that eradicate oxidized mitochondrial proteins. This helps monitor the quality of the mitochondria as the disregulation of its quality control has been linked to cancer and degenerative diseases. This example may contribute to illustrations of the complexity and scope of relatively recent evolution to Homo sapiens.
A change in gene TKTL1 could be a key factor of recent brain evolution and difference of modern humans to (other) apes and Neanderthals, related to neocortex-neurogenesis. However, the "archaic" allele attributed to Neanderthals is present in 0.03% of Homo sapiens, but no resultant phenotypic differences have been reported in these people. Additionally, as Herai et al. contend, more is not always better. In fact, enhanced neuron production "can lead to an abnormally enlarged cortex and layer-specific imbalances in glia/neuron ratios and neuronal subpopulations during neurodevelopment." Even the original study's authors agree that "any attempt to discuss prefrontal cortex and cognitive advantage of modern humans over Neandertals based on TKTL1 alone is problematic".
Some of the prior study's authors reported a similar ARHGAP11B mutation in 2016.
Epigenetics also play a major role in the brain evolution in and to humans.

Recently evolved traits

Language

A genome-wide association study meta-analysis reported genetic factors of, the so far uniquely human, language-related capacities, in particular factors of differences in skill-levels of five tested traits. It e.g. identified association with neuroanatomy of a language-related brain area via neuroimaging correlation. The data contributes to identifying or understanding the biological basis of this recently evolved characteristic capability.

Human brain

One of the prominent ways of tracking the evolution of the human brain is through direct evidence in the form of fossils. The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominids and finally to Homo sapiens. Because fossilized brain tissue is rare, a more reliable approach is to observe anatomical characteristics of the skull that offer insight into brain characteristics. One such method is to observe the endocranial cast (also referred to as endocasts). Endocasts occur when, during the fossilization process, the brain deteriorates away, leaving a space that is filled by surrounding sedimentary material over time. These casts, give an imprint of the lining of the brain cavity, which allows a visualization of what was there. This approach, however, is limited in regard to what information can be gathered. Information gleaned from endocasts is primarily limited to the size of the brain (cranial capacity or endocranial volume), prominent sulci and gyri, and size of dominant lobes or regions of the brain. While endocasts are extremely helpful in revealing superficial brain anatomy, they cannot reveal brain structure, particularly of deeper brain areas. By determining scaling metrics of cranial capacity as it relates to total number of neurons present in primates, it is also possible to estimate the number of neurons through fossil evidence.

Despite the limitations to endocasts, they can and do provide a basis for understanding human brain evolution, which shows primarily a gradually bigger brain. The evolutionary history of the human brain shows primarily a gradually bigger brain relative to body size during the evolutionary path from early primates to hominins and finally to Homo sapiens. This trend that has led to the present day human brain size indicates that there has been a 2-3 factor increase in size over the past 3 million years.^[49] This can be visualized with current data on hominin evolution, starting with Australopithecus, a group of hominins from which humans are likely descended. After all of the data, all observations concluded that the main development that occurred during evolution was the increase of brain size.

However, recent research has called into question the hypothesis of a threefold increase in brain size when comparing Homo sapiens with Australopithecus and chimpanzees. For example, in an article published in 2022 compiled a large data set of contemporary humans and found that the smallest human brains are less than twice that of large brained chimpanzees. As the authors write '...the upper limit of chimpanzee brain size is 500g/ml yet numerous modern humans have brain size below 900 g/ml.' (Note that in this quote, the unit g/ml is to be understood not in the usual way as gram per millilitre but rather as gram or millilitre. This is consistent because brain density is close to 1 g/ml.) Consequently, the authors argue that the notion of an increase in brain size being related to advances in cognition needs to be re-thought in light of global variation in brain size, as the brains of many modern humans with normal cognitive capacities are only 400g/ml larger than chimpanzees. Additionally, much of the increase in brain size - which occurs to a much greater degree in specific modern populations - can be explained by increases in correlated body size related to diet and climatic factors.

Australopiths lived from 3.85 to 2.95 million years ago with the general cranial capacity somewhere near that of the extant chimpanzee—around 300–500 cm³. Considering that the volume of the modern human brain is around 1,352 cm³ on average this represents a substantial amount of brain mass evolved. Australopiths are estimated to have a total neuron count of ~30-35 billion.

Progressing along the human ancestral timeline, brain size continues to steadily increase (see Homininae) when moving into the era of Homo. For example, Homo habilis, living 2.4 million to 1.4 million years ago and argued to be the first Homo species based on a host of characteristics, had a cranial capacity of around 600 cm³. Homo habilis is estimated to have had ~40 billion neurons.^[50]

A little closer to present day, Homo heidelbergensis lived from around 700,000 to 200,000 years ago and had a cranial capacity of around 1290 cm³ and having around 76 billion neurons.

Homo neaderthalensis, living 400,000 to 40,000 years ago, had a cranial capacity comparable to that of modern humans at around 1500–1600 cm³on average, with some specimens of Neanderthal having even greater cranial capacity. Neanderthals are estimated to have had around 85 billion neurons. The increase in brain size topped with Neanderthals, possibly due to their larger visual systems.

It is also important to note that the measure of brain mass or volume, seen as cranial capacity, or even relative brain size, which is brain mass that is expressed as a percentage of body mass, are not a measure of intelligence, use, or function of regions of the brain. Total neurons, however, also do not indicate a higher ranking in cognitive abilities. Elephants have a higher number of total neurons (257 billion) compared to humans (100 billion). Relative brain size, overall mass, and total number of neurons are only a few metrics that help scientists follow the evolutionary trend of increased brain to body ratio through the hominin phylogeny.

In 2021, scientists suggested that the brains of early Homo from Africa and Dmanisi, Georgia, Western Asia "retained a great ape-like structure of the frontal lobe" for far longer than previously thought – until about 1.5 million years ago. Their findings imply that Homo first dispersed out of Africa before human brains evolved to roughly their modern anatomical structure in terms of the location and organization of individual brain regions. It also suggests that this evolution occurred – not during – but only long after the Homo lineage evolved ~2.5 million years ago and after they – Homo erectus in particular – evolved to walk upright. What is the least controversial is that the brain expansion started about 2.6 Ma (about the same as the start of the Pleistocene), and ended around 0.2 Ma.

Evolution of the neocortex

In addition to just the size of the brain, scientists have observed changes in the folding of the brain, as well as in the thickness of the cortex. The more convoluted the surface of the brain is, the greater the surface area of the cortex which allows for an expansion of cortex. It is the most evolutionarily advanced part of the brain. Greater surface area of the brain is linked to higher intelligence as is the thicker cortex but there is an inverse relationship—the thicker the cortex, the more difficult it is for it to fold. In adult humans, thicker cerebral cortex has been linked to higher intelligence.

The neocortex is the most advanced and most evolutionarily young part of the human brain. It is six layers thick and is only present in mammals. It is especially prominent in humans and is the location of most higher level functioning and cognitive ability. The six-layered neocortex found in mammals is evolutionarily derived from a three-layer cortex present in all modern reptiles. This three-layer cortex is still conserved in some parts of the human brain such as the hippocampus and is believed to have evolved in mammals to the neocortex during the transition between the Triassic and Jurassic periods. After looking at history, the mammals had little neocortex compared to the primates as they had more cortex. The three layers of this reptilian cortex correlate strongly to the first, fifth and sixth layers of the mammalian neocortex. Across species of mammals, primates have greater neuronal density compared to rodents of similar brain mass and this may account for increased intelligence.

Theories of human brain evolution

Explanations of the rapid evolution and exceptional size of the human brain can be classified into five groups: instrumental, social, environmental, dietary, and anatomo-physiological. The instrumental hypotheses are based on the logic that evolutionary selection for larger brains is beneficial for species survival, dominance, and spread, because larger brains facilitate food-finding and mating success. The social hypotheses suggest that social behavior stimulates evolutionary expansion of brain size. Similarly, the environmental hypotheses suppose that encephalization is promoted by environmental factors such as stress, variability, and consistency. The dietary theories maintain that food quality and certain nutritional components directly contributed to the brain growth in the Homo genus. The anatomo-physiologic concepts, such as cranio-cerebral vascular hypertension due to head-down posture of the anthropoid fetus during pregnancy, are primarily focused on anatomic-functional changes that predispose to brain enlargement.

No single theory can completely account for human brain evolution. Multiple selective pressures in combination seems to have been involved. Synthetic theories have been proposed, but have not clearly explained reasons for the uniqueness of the human brain. Puzzlingly, brain enlargement has been found to have occurred independently in different primate lineages, but only human lineage ended up with an exceptional brain capacity. Fetal head-down posture may be an explanation of this conundrum because Homo sapiens is the only primate obligatory biped with upright posture.