The March of Time: Evolving Conceptions of Time in the Light of Scientific Discoveries

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When a non-native species arrives in an ecosystem, the native species often are ill-equipped to defend against the foreign invader. As a result, tens of millions of ash trees across the continent are threatened by the green beetle, which is thought to have hitchhiked to the U. Advances in molecular biology allow them to identify the very genes that help individuals move away from their ancestors as they adapt to new conditions.

For example, a team of scientists at Harvard Medical School and Princeton identified BMP4 as the key gene that sculpts the beaks of the Galapagos finches that gave Darwin his first inklings of the theory of evolution. Reznick and his team continue to transplant guppies in the tropical streams of Trinidad. Along with other new tools of evolutionary biology, such as DNA analysis, they are using a technique Reznick developed to mark individual guppies so they can more easily recapture them later.

This marking method allows his team to reconstruct the pedigree of individual guppies and measure their reproductive success. The research is answering questions about how predator-prey interactions shape evolving species, but it has also generated new questions: If environmental forces can change guppies, could the guppies themselves also change their ecosystems? Intrigued, Reznick has turned his attention to how ecological and evolutionary processes interact.

His team wants to know how guppies adapt to predators as well as how predators evolve in response to guppies — the ecological cause-and-effect relationships. This raises fundamental questions about ecological restoration, invasive species, natural selection and what else changes when the fittest actually survive.

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Flex your cortex with Discover. Discover Magazine on Facebook Discover Magazine. Though the arguments he used were lost, Plutarch stated that Seleucus was the first to prove the heliocentric system through reasoning. A leading scientist of classical antiquity, Archimedes also developed elaborate systems of pulleys to move large objects with a minimum of effort. The Archimedes' screw underpins modern hydroengineering, and his machines of war helped to hold back the armies of Rome in the First Punic War.

Archimedes even tore apart the arguments of Aristotle and his metaphysics, pointing out that it was impossible to separate mathematics and nature and proved it by converting mathematical theories into practical inventions. In mathematics, Archimedes used the method of exhaustion to calculate the area under the arc of a parabola with the summation of an infinite series, and gave a remarkably accurate approximation of pi.

He also defined the spiral bearing his name , formulae for the volumes of surfaces of revolution and an ingenious system for expressing very large numbers. He also developed the principles of equilibrium states and centers of gravity , ideas that would influence the well known scholars, Galileo, and Newton. Hipparchus — BCE , focusing on astronomy and mathematics, used sophisticated geometrical techniques to map the motion of the stars and planets , even predicting the times that Solar eclipses would happen.

In addition, he added calculations of the distance of the Sun and Moon from the Earth, based upon his improvements to the observational instruments used at that time. Another of the most famous of the early physicists was Ptolemy 90— CE , one of the leading minds during the time of the Roman Empire. Ptolemy was the author of several scientific treatises, at least three of which were of continuing importance to later Islamic and European science.

The second is the Geography , which is a thorough discussion of the geographic knowledge of the Greco-Roman world. Much of the accumulated knowledge of the ancient world was lost. Even of the works of the better known thinkers, few fragments survived. Although he wrote at least fourteen books, almost nothing of Hipparchus ' direct work survived.

Of the reputed Aristotelian works, only 30 exist, and some of those are "little more than lecture notes" [ according to whom? Important physical and mathematical traditions also existed in ancient Chinese and Indian sciences. In Indian philosophy , Maharishi Kanada was the first to systematically develop a theory of atomism around BCE [3] though some authors have allotted him an earlier era in the 6th century BCE.

These philosophers believed that other elements except ether were physically palpable and hence comprised minuscule particles of matter. The last minuscule particle of matter that could not be subdivided further was termed Parmanu. These philosophers considered the atom to be indestructible and hence eternal.

Evolution #2: “Common Descent” or “Universal Common Descent”

The Buddhists thought atoms to be minute objects unable to be seen to the naked eye that come into being and vanish in an instant. The Vaisheshika school of philosophers believed that an atom was a mere point in space.

Evolution #1: “Change over Time”

Darwin, in Origin , admitted that the lack of "intermediate varieties" in the geological record was "the most obvious and gravest objection which can be urged against my theory. In any case, where theistic evolution is understood to affirm the third meaning of evolution—the creative power and adequacy of the neo-Darwinian mechanism and its consequent denial of actual design—the concept becomes deeply problematic. Consequently, he suggests that people of faith who yield core convictions about the intelligent design of life—out of deference to the supposed scientific authority of spokesmen for Darwinism—do so unnecessarily and with a substantial apologetic cost to their faith. She doesn't shy away from controversial topics, either — she tackles the topics of asteroid mining and asteroid collision with factuality, openness, and ease. Nature itself could play the role of the breeder and, thus, eliminate the need for an actual designing intelligence to produce the complex adaptations that living organisms manifest. Mammals now live in a world that was once dominated by reptilian giants such as Tyrannosaurus rex. Reznick and his team continue to transplant guppies in the tropical streams of Trinidad.

It was also first to depict relations between motion and force applied. Indian theories about the atom are greatly abstract and enmeshed in philosophy as they were based on logic and not on personal experience or experimentation. In Indian astronomy , Aryabhata 's Aryabhatiya CE proposed the Earth's rotation , while Nilakantha Somayaji — of the Kerala school of astronomy and mathematics proposed a semi-heliocentric model resembling the Tychonic system.

In optics, Shen Kuo independently developed a camera obscura. In the 7th to 15th centuries, scientific progress occurred in the Muslim world. Many classic works in Indian , Assyrian , Sassanian Persian and Greek , including the works of Aristotle , were translated into Arabic. Ptolemy and Aristotle theorised that light either shone from the eye to illuminate objects or that "forms" emanated from objects themselves, whereas al-Haytham known by the Latin name "Alhazen" suggested that light travels to the eye in rays from different points on an object.

Ibn al-Haytham and Biruni were early proponents of the scientific method. Ibn al-Haytham is considered to be the "father of the modern scientific method" due to his emphasis on experimental data and reproducibility of its results. He published his theory of motion in Book of Healing , where he argued that an impetus is imparted to a projectile by the thrower, and believed that it was a temporary virtue that would decline even in a vacuum. He viewed it as persistent, requiring external forces such as air resistance to dissipate it.

He concluded that continuation of motion is attributed to the inclination that is transferred to the object, and that object will be in motion until the mayl is spent. He also claimed that projectile in a vacuum would not stop unless it is acted upon. This conception of motion is consistent with Newton's first law of motion , inertia , which states that an object in motion will stay in motion unless it is acted on by an external force. He used this to compose a calendar considered more accurate than the Gregorian calendar that came along years later.

Hibat Allah Abu'l-Barakat al-Baghdaadi c. In his Kitab al-Mu'tabar , Abu'l-Barakat stated that the mover imparts a violent inclination mayl qasri on the moved and that this diminishes as the moving object distances itself from the mover. Ibn Bajjah c. While he did not specify that these forces be equal, it was a precursor to Newton's third law of motion which states that for every action there is an equal and opposite reaction.

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Two future philosophers supported the theories Avempace created, known as Avempacean dynamics. Nasir al-Din al-Tusi — , a Persian astronomer and mathematician who died in Baghdad, authored the Treasury of Astronomy , a remarkably accurate table of planetary movements that reformed the existing planetary model of Roman astronomer Ptolemy by describing a uniform circular motion of all planets in their orbits.

This work led to the later discovery, by one of his students, that planets actually have an elliptical orbit.

Awareness of ancient works re-entered the West through translations from Arabic to Latin. Their re-introduction, combined with Judeo-Islamic theological commentaries, had a great influence on Medieval philosophers such as Thomas Aquinas. Scholastic European scholars , who sought to reconcile the philosophy of the ancient classical philosophers with Christian theology , proclaimed Aristotle the greatest thinker of the ancient world.

In cases where they didn't directly contradict the Bible, Aristotelian physics became the foundation for the physical explanations of the European Churches. Quantification became a core element of medieval physics. Based on Aristotelian physics, Scholastic physics described things as moving according to their essential nature.

Celestial objects were described as moving in circles, because perfect circular motion was considered an innate property of objects that existed in the uncorrupted realm of the celestial spheres. The theory of impetus , the ancestor to the concepts of inertia and momentum , was developed along similar lines by medieval philosophers such as John Philoponus and Jean Buridan. Motions below the lunar sphere were seen as imperfect, and thus could not be expected to exhibit consistent motion.

Evolution #2: “Common Descent” or “Universal Common Descent”

More idealized motion in the "sublunary" realm could only be achieved through artifice , and prior to the 17th century, many did not view artificial experiments as a valid means of learning about the natural world. Physical explanations in the sublunary realm revolved around tendencies. Stones contained the element earth, and earthly objects tended to move in a straight line toward the centre of the earth and the universe in the Aristotelian geocentric view unless otherwise prevented from doing so.

During the 16th and 17th centuries, a large advancement of scientific progress known as the Scientific revolution took place in Europe. Dissatisfaction with older philosophical approaches had begun earlier and had produced other changes in society, such as the Protestant Reformation , but the revolution in science began when natural philosophers began to mount a sustained attack on the Scholastic philosophical programme and supposed that mathematical descriptive schemes adopted from such fields as mechanics and astronomy could actually yield universally valid characterizations of motion and other concepts.

A breakthrough in astronomy was made by Polish astronomer Nicolaus Copernicus — when, in , he gave strong arguments for the heliocentric model of the Solar system , ostensibly as a means to render tables charting planetary motion more accurate and to simplify their production. In heliocentric models of the Solar system, the Earth orbits the Sun along with other bodies in Earth's galaxy , a contradiction according to the Greek-Egyptian astronomer Ptolemy 2nd century CE; see above , whose system placed the Earth at the center of the Universe and had been accepted for over 1, years.

The Greek astronomer Aristarchus of Samos c. Copernicus' book presenting the theory De revolutionibus orbium coelestium , "On the Revolutions of the Celestial Spheres" was published just before his death in and, as it is now generally considered to mark the beginning of modern astronomy, is also considered to mark the beginning of the Scientific revolution. The Italian mathematician, astronomer, and physicist Galileo Galilei — was the central figure in the Scientific revolution and famous for his support for Copernicanism, his astronomical discoveries, empirical experiments and his improvement of the telescope.

As a mathematician, Galileo's role in the university culture of his era was subordinated to the three major topics of study: law , medicine , and theology which was closely allied to philosophy. Galileo's early studies at the University of Pisa were in medicine, but he was soon drawn to mathematics and physics. At 19, he discovered and, subsequently, verified the isochronal nature of the pendulum when, using his pulse, he timed the oscillations of a swinging lamp in Pisa's cathedral and found that it remained the same for each swing regardless of the swing's amplitude.

He soon became known through his invention of a hydrostatic balance and for his treatise on the center of gravity of solid bodies. While teaching at the University of Pisa —92 , he initiated his experiments concerning the laws of bodies in motion that brought results so contradictory to the accepted teachings of Aristotle that strong antagonism was aroused.

He found that bodies do not fall with velocities proportional to their weights. The famous story in which Galileo is said to have dropped weights from the Leaning Tower of Pisa is apocryphal, but he did find that the path of a projectile is a parabola and is credited with conclusions that anticipated Newton's laws of motion e. Among these is what is now called Galilean relativity , the first precisely formulated statement about properties of space and time outside three-dimensional geometry.

Galileo has been called the "father of modern observational astronomy ", [29] the "father of modern physics ", [30] the "father of science", [30] and "the father of modern science ". Found "vehemently suspect of heresy", he was forced to recant and spent the rest of his life under house arrest.

The contributions that Galileo made to observational astronomy include the telescopic confirmation of the phases of Venus ; his discovery, in , of Jupiter's four largest moons subsequently given the collective name of the " Galilean moons " ; and the observation and analysis of sunspots. Galileo also pursued applied science and technology, inventing, among other instruments, a military compass.

His discovery of the Jovian moons was published in and enabled him to obtain the position of mathematician and philosopher to the Medici court. As such, he was expected to engage in debates with philosophers in the Aristotelian tradition and received a large audience for his own publications such as the Discourses and Mathematical Demonstrations Concerning Two New Sciences published abroad following his arrest for the publication of Dialogue Concerning the Two Chief World Systems and The Assayer.

This tradition, combining with the non-mathematical emphasis on the collection of "experimental histories" by philosophical reformists such as William Gilbert and Francis Bacon , drew a significant following in the years leading up to and following Galileo's death, including Evangelista Torricelli and the participants in the Accademia del Cimento in Italy; Marin Mersenne and Blaise Pascal in France; Christiaan Huygens in the Netherlands; and Robert Hooke and Robert Boyle in England.

Descartes had a more ambitious agenda, however, which was geared toward replacing the Scholastic philosophical tradition altogether. Questioning the reality interpreted through the senses, Descartes sought to re-establish philosophical explanatory schemes by reducing all perceived phenomena to being attributable to the motion of an invisible sea of "corpuscles". Notably, he reserved human thought and God from his scheme, holding these to be separate from the physical universe.

In proposing this philosophical framework, Descartes supposed that different kinds of motion, such as that of planets versus that of terrestrial objects, were not fundamentally different, but were merely different manifestations of an endless chain of corpuscular motions obeying universal principles. Particularly influential were his explanations for circular astronomical motions in terms of the vortex motion of corpuscles in space Descartes argued, in accord with the beliefs, if not the methods, of the Scholastics, that a vacuum could not exist , and his explanation of gravity in terms of corpuscles pushing objects downward.

Descartes, like Galileo, was convinced of the importance of mathematical explanation, and he and his followers were key figures in the development of mathematics and geometry in the 17th century. Cartesian mathematical descriptions of motion held that all mathematical formulations had to be justifiable in terms of direct physical action, a position held by Huygens and the German philosopher Gottfried Leibniz , who, while following in the Cartesian tradition, developed his own philosophical alternative to Scholasticism, which he outlined in his work, The Monadology.

Descartes has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day. In particular, his Meditations on First Philosophy continues to be a standard text at most university philosophy departments. Descartes' influence in mathematics is equally apparent; the Cartesian coordinate system — allowing algebraic equations to be expressed as geometric shapes in a two-dimensional coordinate system — was named after him.

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He is credited as the father of analytical geometry , the bridge between algebra and geometry , important to the discovery of calculus and analysis. The late 17th and early 18th centuries saw the achievements of the greatest figure of the Scientific revolution: Cambridge University physicist and mathematician Sir Isaac Newton , considered by many to be the greatest and most influential scientist who ever lived. Newton, a fellow of the Royal Society of England , combined his own discoveries in mechanics and astronomy to earlier ones to create a single system for describing the workings of the universe.

Newton formulated three laws of motion which formulated the relationship between motion and objects and also the law of universal gravitation , the latter of which could be used to explain the behavior not only of falling bodies on the earth but also planets and other celestial bodies. To arrive at his results, Newton invented one form of an entirely new branch of mathematics: calculus also invented independently by Gottfried Leibniz , which was to become an essential tool in much of the later development in most branches of physics.

Newton was able to refute the Cartesian mechanical tradition that all motions should be explained with respect to the immediate force exerted by corpuscles. Using his three laws of motion and law of universal gravitation, Newton removed the idea that objects followed paths determined by natural shapes and instead demonstrated that not only regularly observed paths, but all the future motions of any body could be deduced mathematically based on knowledge of their existing motion, their mass , and the forces acting upon them.

However, observed celestial motions did not precisely conform to a Newtonian treatment, and Newton, who was also deeply interested in theology , imagined that God intervened to ensure the continued stability of the solar system. Newton's principles but not his mathematical treatments proved controversial with Continental philosophers, who found his lack of metaphysical explanation for movement and gravitation philosophically unacceptable. Beginning around , a bitter rift opened between the Continental and British philosophical traditions, which were stoked by heated, ongoing, and viciously personal disputes between the followers of Newton and Leibniz concerning priority over the analytical techniques of calculus , which each had developed independently.

Initially, the Cartesian and Leibnizian traditions prevailed on the Continent leading to the dominance of the Leibnizian calculus notation everywhere except Britain. Newton himself remained privately disturbed at the lack of a philosophical understanding of gravitation while insisting in his writings that none was necessary to infer its reality.

As the 18th century progressed, Continental natural philosophers increasingly accepted the Newtonians' willingness to forgo ontological metaphysical explanations for mathematically described motions. Newton built the first functioning reflecting telescope [41] and developed a theory of color, published in Opticks , based on the observation that a prism decomposes white light into the many colours forming the visible spectrum. While Newton explained light as being composed of tiny particles, a rival theory of light which explained its behavior in terms of waves was presented in by Christiaan Huygens.

However, the belief in the mechanistic philosophy coupled with Newton's reputation meant that the wave theory saw relatively little support until the 19th century. Newton also formulated an empirical law of cooling , studied the speed of sound , investigated power series , demonstrated the generalised binomial theorem and developed a method for approximating the roots of a function. His work on infinite series was inspired by Simon Stevin 's decimals. By demonstrating the consistency between Kepler's laws of planetary motion and his own theory of gravitation, Newton also removed the last doubts about heliocentrism.

By bringing together all the ideas set forth during the Scientific revolution, Newton effectively established the foundation for modern society in mathematics and science. Other branches of physics also received attention during the period of the Scientific revolution.

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William Gilbert , court physician to Queen Elizabeth I , published an important work on magnetism in , describing how the earth itself behaves like a giant magnet. Robert Boyle —91 studied the behavior of gases enclosed in a chamber and formulated the gas law named for him ; he also contributed to physiology and to the founding of modern chemistry. Another important factor in the scientific revolution was the rise of learned societies and academies in various countries.

The earliest of these were in Italy and Germany and were short-lived. The former was a private institution in London and included such scientists as John Wallis , William Brouncker , Thomas Sydenham , John Mayow , and Christopher Wren who contributed not only to architecture but also to astronomy and anatomy ; the latter, in Paris, was a government institution and included as a foreign member the Dutchman Huygens.

In the 18th century, important royal academies were established at Berlin and at St. Petersburg The societies and academies provided the principal opportunities for the publication and discussion of scientific results during and after the scientific revolution. In , James Bernoulli showed that the cycloid is the solution to the tautochrone problem; and the following year, in , Johann Bernoulli showed that a chain freely suspended from two points will form a catenary , the curve with the lowest possible center of gravity available to any chain hung between two fixed points.

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The aim of this interdisciplinary study is to reconstruct the evolution of our changing conceptions of time in the light of scientific discoveries. It will adopt a new. The March of Time: Evolving Conceptions of Time in the Light of Scientific Discoveries. Berlin/Heidelberg: Springer-Verlag, pp.

He then showed, in , that the cycloid is the solution to the brachistochrone problem. A precursor of the engine was designed by the German scientist Otto von Guericke who, in , designed and built the world's first vacuum pump and created the world's first ever vacuum known as the Magdeburg hemispheres experiment. He was driven to make a vacuum to disprove Aristotle 's long-held supposition that 'Nature abhors a vacuum'. Shortly thereafter, Irish physicist and chemist Boyle had learned of Guericke's designs and in , in coordination with English scientist Robert Hooke , built an air pump.

In that time, air was assumed to be a system of motionless particles, and not interpreted as a system of moving molecules. The concept of thermal motion came two centuries later. Therefore, Boyle's publication in speaks about a mechanical concept: the air spring. This tool gave Gay-Lussac the opportunity to derive his law , which led shortly later to the ideal gas law.

But, already before the establishment of the ideal gas law, an associate of Boyle's named Denis Papin built in a bone digester, which is a closed vessel with a tightly fitting lid that confines steam until a high pressure is generated. Later designs implemented a steam release valve to keep the machine from exploding. By watching the valve rhythmically move up and down, Papin conceived of the idea of a piston and cylinder engine. He did not however follow through with his design. Nevertheless, in , based on Papin's designs, engineer Thomas Savery built the first engine.

Although these early engines were crude and inefficient, they attracted the attention of the leading scientists of the time. Hence, prior to and the invention of the Savery Engine , horses were used to power pulleys, attached to buckets, which lifted water out of flooded salt mines in England. In the years to follow, more variations of steam engines were built, such as the Newcomen Engine , and later the Watt Engine. In time, these early engines would eventually be utilized in place of horses.

Thus, each engine began to be associated with a certain amount of "horse power" depending upon how many horses it had replaced. In other words, large quantities of coal or wood had to be burned to yield only a small fraction of work output. Hence the need for a new science of engine dynamics was born.

During the 18th century, the mechanics founded by Newton was developed by several scientists as more mathematicians learned calculus and elaborated upon its initial formulation. The application of mathematical analysis to problems of motion was known as rational mechanics, or mixed mathematics and was later termed classical mechanics. In , Brook Taylor derived the fundamental frequency of a stretched vibrating string in terms of its tension and mass per unit length by solving a differential equation.

The reward for being a slightly faster or more alert antelope? The lions would eat your slower neighbors first, granting you one more day in which to reproduce. After many generations and a great deal of time, the whole population would run faster, and with many such changes over time eventually become a new species.

Evolution, Darwin's "descent with modification through natural selection," would have occurred. But what was the source of variation and what was the mechanism for passing change from generation to generation? Darwin "didn't know anything about why organisms resemble their parents, or the basis of heritable variations in populations," says Niles Eldredge, a paleontologist at the American Museum of Natural History in New York City.

In Darwin's era, the man who did make progress on the real mechanism of inheritance was the Austrian monk Gregor Mendel. In his abbey garden in the late s and early s, Mendel bred pea plants and found that the transmission of traits such as flower color and seed texture followed observable rules. For instance, when plants with certain distinct traits were bred with each other, the hybrid offspring did not have a trait that was a blend of the two; the flowers might be purple or white, but never an intermediate violet.

This surprising result helped point the way toward the concept of "units" of inheritance—discrete elements of hereditary information. An offspring inherits a set of these genetic units from each parent. Since the early s, those units of inheritance have been known as genes. Mendel knew Darwin's work—his German copy of Origin was sprinkled with handwritten notes—but there's no evidence that Mendel realized that his units of inheritance carried the variation upon which Darwinian selection acted.

But what if he had? Today, comparative genomics—the analysis of whole sets of genetic information from different species—is confirming the core of Darwin's theory at the deepest level. Scientists can now track, DNA molecule by DNA molecule, exactly what mutations occurred, and how one species changed into another. Darwin himself made a stab at drawing a "tree of life," a diagram that traces the evolutionary relationships among species based on their similarities and differences.

There have been plenty of evolutionary surprises in recent years, things that Darwin never would have guessed. The number of genes a species has doesn't correlate with how complex it is, for example. With some 37, genes, rice has almost twice as many as humans, with 20, And genes aren't passed only from parent to offspring; they can also be passed between individuals, even individuals of different species.

This "horizontal transfer" of genetic material is pervasive in bacteria; it's how antibiotic resistance often spreads from one strain to another. Animals rarely acquire whole genes in this way, but our own DNA is packed with smaller bits of genetic material picked up from viruses during our evolutionary history, including many elements that regulate when genes are active or dormant. Do these surprises challenge the central idea of Darwinian evolution? Truly one of the most remarkable traits of Darwinism itself is that it has withstood heavy scientific scrutiny for a century and a half and still manages to accommodate the latest ideas.

Another growing field of biology is shedding further light on the origins of variation. Evolutionary developmental biology, or evo-devo, focuses on changes in the exquisitely choreographed process that causes a fertilized egg to mature. Behind one series of such changes are the so-called homeotic genes, which dictate where legs or arms or eyes will form on a growing embryo. These central-control genes turned out to be almost identical even in animals as different as worms, flies and human beings.

Many researchers now think that much of evolution works not so much through mutations, or random errors, in the major functional genes, but by tweaking the ways by which developmental genes control other genes. These kinds of connections were at the heart of descent with modification. Carroll says he thinks Darwin would be thrilled with the evolutionary details scientists can now see—how, for example, changes in just a small number of regulatory genes can explain the evolution of insects, which have six legs, from their ancestors, which had even more. From there, it's a short step to solving some of the mysteries of speciation, working out the mechanics of exactly how one species becomes many, and how complexity and diversity can be built up out of very simple beginnings.

Perhaps the most surprising discovery in recent years has to do with one of Darwin's predecessors in evolutionary theory. Jean-Baptiste Lamarck, a French naturalist, developed his own theory of biological evolution in the early 19th century.

What Darwin Didn't Know | Science | Smithsonian

He suggested that acquired traits could be passed along to offspring—giraffes that stretched to reach leaves on tall trees would produce longer-necked offspring. This "soft inheritance" became known as Lamarckism and soon proved susceptible to parody: Would clipping the tail off a rat lead to tailless pups? Of course not, and in time soft inheritance was dismissed, and Lamarck became a textbook example of shoddy thinking. Then, in the early days of genetic engineering more than two decades ago, researchers inserted foreign genes into the DNA of lab animals and plants and noticed something strange.

The genes inserted into such host cells worked at first, "but then suddenly they were silenced, and that was it, generation after generation," says Eva Jablonka, an evolutionary biologist at Tel Aviv University in Israel. Researchers figured out that the host cells were tagging the foreign genes with an "off switch" that made the genes inoperable. The new gene was passed to an animal's offspring, but so was the off switch—that is, the parent's experience influenced its offspring's inheritance.

All sorts of changes in cellular machinery have shown up that have nothing to do with the sequence of DNA but still have profound, and heritable, impacts for generations to come. For example, malnourished rats give birth to undersized pups that, even if well fed, grow up to give birth to undersized pups. Which means, among other things, that poor old Lamarck was right—at least some acquired traits can be passed down. Darwin included the concept of soft inheritance in Origin , mentioning "variability from the indirect and direct action of the external conditions of life, and from use and disuse," for example.

It has been said that Darwin himself was not a particularly strict Darwinian, meaning that his work allowed for a wider variety of mechanisms than many of his 20th-century followers would accept. Origin barely touched upon the most contentious evolutionary issue: If all life has evolved from "lower forms," does that include people? Darwin finally addressed the issue in The Descent of Man, and Selection in Relation to Sex , published in , explaining he had been studying human evolution for years, but "with the determination not to publish, as I thought that I should thus only add to the prejudices against my views.

They shared Disraeli's discomfort at being descended from apes and complained that evolution pushed a divine creator to the side. Disbelief in human descent may have been a justifiable comfort in Darwin's time, when few fossils of human ancestors had been discovered, but the evidence no longer allows it. Darwin, in Origin , admitted that the lack of "intermediate varieties" in the geological record was "the most obvious and gravest objection which can be urged against my theory.

The objection certainly applied to the paucity of ancestral human fossils in Darwin's time. Years of painstaking work by paleontologists, however, have filled in many of the important gaps. There are many more extinct species to be discovered, but the term "missing link" has for the most part become as outdated as the idea of special creation for each species. Anthropologists once depicted human evolution as a version of the classic "March of Progress" image—a straight line from a crouching proto-ape, through successive stages of knuckle draggers and culminating in upright modern human beings.

There are now hundreds of known fossils, stretching back six to seven million years and representing about two dozen species. Some were our ancestors and others distant cousins. Remarkably, our modern human forebears shared parts of Europe and western Asia with the Neanderthal species as recently as 30, years ago, and they may have also overlapped with two other long-gone ancient humans, Homo floresiensis and Homo erectus , in Southeast Asia.

Darwin himself was confident that the deep past would be revealed. Asked about gaps in Darwin's knowledge, Francisco Ayala, a biologist at the University of California at Irvine, laughs.