26 Mar 08
Leonardo made two seminal contributions to the understanding of evolution. The first was an accurate description of fossil formation and the second his recognition of anatomical similarities among animals and humans, an early precursor to the field of comparative anatomy. Both findings would be important in presenting evidence for evolution many hundreds of years later. His findings, however, would lie undiscovered with little impact on succeeding scientific thought until long after his death.
But before we go into that it is interesting, I think, to understand the prevailing conditions in his time, if only to better appreciate his incredible contribution.
Long before Leonardo, during what are now called the Dark Ages, which extended from about 500-1000 CE after the fall of the Roman Empire, Greek and Roman science was largely forgotten in Western Europe. By the sixth century AD the scientific traditions of Ancient Greece had largely petered out, but the Greeks had left behind important texts. Philosophical and scientific teaching during this time was based upon the few copies and commentaries of these ancient Greek texts that remained in Western Europe after the collapse of the Western Roman Empire.
In the seventh century, powerful Muslim armies inspired by the new religion of Islam, in successive invasions conquered the peoples of the Middle East, North Africa and Southern Europe. As they built their vast empire, they not only spread Islam and Arabic, but also appreciated the Greek learning they came across in the ancient texts of Greek Science and philosophy in the Byzantine libraries. The Arabs translated all the important philosophical and scientific works into Arabic and assimilated much of the science of Antiquity into their culture.
In contrast to the Romans, Arab scholars not only assimilated the Greek knowledge but added to it their learning from other cultures such as from the Far East as well as their own commentaries and interpretations, further advancing the gains made by Greeks. New versions of ancient texts with added knowledge were housed in huge libraries across the Islamic empire. Towards the tail end of the Islamic influence, in Moorish Spain for example, the great libraries of Cordoba alone contained some six hundred thousand manuscripts!
For two centuries between 750 and 950, the caliphs of the Abbasid dynasty, centred in Baghdad, supported a mammoth translation effort. Greek works translated into Arabic include Euclid’s geometry, Ptolemy’s astronomy, the medical works of Galen and Hippocrates and the pharmacopoeia of Dioscorides. The caliphs understood the importance of scholarship to their expanding empire. As the reach of the Islamic world spread, stretching from northern India to Spain, they absorbed as much knowledge as they could from each conquest. In Persia and India, unlike Greece, the scientific traditions were still very much alive. And so it was to Persian and Indian scholars that the translators turned when they needed to make scientific, as well as linguistic, sense of the old Greek manuscripts (see previous post). Such expertise was vital because the caliphs wanted their acquired knowledge to deliver practical as well as intellectual benefits to their empire: from monumental architecture and city planning to medical care and transport.
Baghdad, as the capital of the Abbasid empire had been a center of learning for the 8th and 9th centures but saw the beginnings of a decline starting at the end of this period. The city’s early meteoric growth slowed due to troubles within the Caliphate, including relocations of the capital to Samarra (during 808–819 and 836–892), the loss of the western and easternmost provinces, and periods of political domination by the Iranian Buwayhids (945–1055) and Seljuk Turks (1055–1135). Nevertheless, the city remained one of the cultural and commercial hubs of the Islamic world until February 10, 1258, when it was sacked by the Mongols under Hulagu Khan. The Mongols massacred most of the city’s inhabitants, including the Abbasid Caliph Al-Musta’sim, and destroyed large sections of the city including its libraries and centers of learning. The sack of Baghdad put an end to the Abbasid Caliphate, and caused some scholars to move west and Spain became the greatest center of Islamic learning and technical knowledge. This moving to a more westerly location would have dramatic consequences for Christian Europe as we will see later.
At around the same time, the great Buddhist university of Nalanda that had operated for over five hundred years from 427 CE to 1197 CE was sacked by Muslim armies under the Pashtun/Turkic invader Bakhtiyar Khilji in 1193. This event is seen as a milestone in the decline of Buddhism in India. In contrast to the Arabs these invaders cared little for the university or the fact that it was a leading center of learning and a majority of the historic texts and scrolls in the university were destroyed at this time.
Nalanda had been one of the world’s first residential universities with dormitories for over 10,000 students and 2,000 teachers. The university was considered an architectural masterpiece, and was marked by a lofty wall and one gate. On the grounds were lakes and parks. The library was located in a nine storied building where meticulous copies of texts were produced. The subjects taught at Nalanda University covered every field of learning from science, astronomy, medicine, and logic to metaphysics, philosophy, Samkhya, Yoga-shastra, the Vedas, and the scriptures of Buddhism. The university attracted pupils and scholars from Korea, Japan, China, Tibet, Indonesia, Persia and Turkey. The Tang Dynasty Chinese pilgrim Xuanzang left detailed accounts of the university in the 7th century. The destruction of the temples, monasteries, centers of learning at Nalanda and northern India were probably responsible for the demise of ancient Indian scientific thought in mathematics, astronomy, alchemy, and anatomy.
As Nalanda and Baghdad died, the 12th century in Europe began an age of fresh and vigorous life: the epoch of the Crusades, of the rise of towns, and of the earliest bureaucratic states of the West, it saw the culmination of Romanesque art and the beginnings of Gothic; the emergence of the vernacular literatures; the revival of the Latin classics and of Latin poetry and Roman law; the recovery of Greek science, with its Arabic additions, and of much of Greek philosophy; and the origin of the first European universities.
Renaissance of the 12th century, the increased contact with the Islamic world
in Spain and Sicily, the Crusades, the Reconquista, as well as increased
contact with Byzantium, allowed Europeans to seek and translate
the works of Hellenic and Islamic philosophers and scientists, especially the
works of Aristotle, Euclid, Ptolemy, Plotinus, Geber, al-Khwarizmi, Rhazes,
Abulcasis, Alhacen, Avicenna, Avempace, and Averroes, among others.
The Renaissance was so called because it was a “rebirth” of certain classical ideas that had long been lost to Europe. It has been argued that the fuel for this rebirth was the rediscovery of ancient texts that had been forgotten by Western civilization, but were preserved in some monastic libraries and in the Islamic world, and the translations of Greek and Arabic texts into Latin. Renaissance scholars such as Niccolò de’ Niccoli and Poggio Bracciolini scoured the libraries of Europe in search of works by such classical authors as Plato, Cicero and Vitruvius. Additionally, as the reconquest of the Iberian peninsula from Islamic Moors progressed, numerous Greek and Arabic works were captured from educational institutions such as the library at Córdoba, mentioned above. The works of ancient Greek and Hellenistic writers (such as Plato, Aristotle, Euclid, Ptolemy, and Plotinus) and Muslim scientists and philosophers (such as Geber, Abulcasis, Alhacen, Avicenna, Avempace, and Averroes), were imported into the Christian world, providing new intellectual material for European scholars. Particularly in the case of mathematical knowledge, some of the work of Muslim scholars was itself a compilation or translation of the earlier work of Indian mathematicians.
Greek and Arabic knowledge was not only assimilated from Spain, but also directly from the Middle East. The study of mathematics was flourishing in the Middle East, and mathematical knowledge was brought back by crusaders in the 13th century. The decline of the Byzantine Empire after 1204 - and its eventual fall in 1453 - led to a sharp increase in the exodus of Greek scholars to Italy and beyond. These scholars brought with them texts and knowledge of the classical Greek civilization which had been lost for centuries in the West and they transmitted the art of exegesis.
By the fifteenth century, even the western outposts of the Islamic world had shrunk considerably under military pressure from Western Europe — the last Muslim forces were forced out of Spain in 1492, the year Christopher Columbus reached America. When the Christian armies won territory during he crusades, their spoils often included the works of Arab scholars. Among the treasures left behind by the Moors in Toledo was one of the finest Islamic libraries filled with precious Arabic translations of Greek scientific and philosophical texts. The occupying Christian forces included Christian monks who set about translating the ancient texts into Latin. A hundred years later this learning was available to the west once again.
As the European Renaissance got under way this accumulated Islamic knowledge was sucked up by powers on the rise, such as Spain and France. Many Arabic works had by then been translated into Latin, but the sources themselves were neglected. Although European libraries and museums collected Arabic scripts, they sat in obscurity as they were largely indecipherable. Over time these trophies of ancient Islam were taken even farther afield to Russia and the United States. India, formerly under Islamic rule, is estimated to have 50,000 such manuscripts.
It was into this environment that Leonardo da Vinci who was fond of describing himself as omo sanza lettere (“an unlettered man”) arrived on April 15, 1452. Born as the illegitimate son of an ambitious and later well connected notary, Piero da Vinci, and a peasant girl, Caterina, Lenoardo spent his early days in Vinci in the company of his uncle Francesco. Francesco, only sixteen years older than Leonardo inculcated in him an understanding and respect for the nature around. Although Leonardo attended one of the customary sculo d’abasco which taught children how to read, write and enough mathematics to survive as a merchant, as an illegitimate child he would be denied entry to a scuola di lettere, a prerequisite to university.
Leonardo made up for this lack of formal education by striving constantly to educate himself in his later life. He learnt Latin to access the manuscripts in the libraries of the time, consulting scholars and assembling a considerable personal library. After his uncle Francesco’s death, Lenoardo moved to Florence to be with his father, who introduced him to his friend, renowned artist and craftsman, Verrocchio. Close to Verrocchio’s workshop was the bottega of the brothers Polliaiolo whose paintings were known for their vivid renderings of muscular bodies. They derived their knowledge of human musculature and anatomy through frequent dissections that it is speculated Leonardo must have watched. At the age of twenty, by then a master painter he was admitted into the guild of painters known as Compagnia di San Luca in 1472. Fortuitously this guild was included in the guild of physicians and apothecaries, which was based at the hospital of Santa Maria Nuova and which allowed him to perform his anatomical dissections.
One of Leonardo’s defining legacies was the synthesis of art and science and he not only brought the knowledge of science to the arts, refining perspective and adding mathematics to drawing, but also brought his keen powers of observation and his skill in the arts to the fields of natural history, anatomy, engineering and architecture.
Now that we have a sense for his time and environs, I would like to return to Leonardo’s two main contributions regarding evolution, the fossil record and comparative anatomy.
Fossils: As I mentioned at the beginning of this article, Leonardo was one of the first scholars to express a relatively modern view of fossils. The fossil record remains first and foremost among the databases that document changes in past life on Earth. Fossils provide the dimension of time to the study of life. Some of the most basic observations about fossils and the rock record were made long before Charles Darwin formulated his theory. The fossil record clearly shows changes in life through almost any sequence of sedimentary rock layers. Successive rock layers contain different groups or assemblages of fossil species.
Leonardo was well acquainted with the rocks and fossils (mostly Cenozoic mollusks) found in his native north Italy having at least observed them during his service as an engineer and artist at the court of Lodovico Sforza, Duke of Milan, from 1482 to 1499. His biographer Vasari wrote that “Leonardo was frequently occupied in the preparation of plans to remove mountains or to pierce them with tunnels from plain to plain.”
Leonardo made many observations on mountains and rivers, and he grasped the principle that rocks can be formed by deposition of sediments by water, while at the same time the rivers erode rocks and carry their sediments to the sea, in a continuous grand cycle. He wrote:
“The stratified stones of the mountains are all layers of clay, deposited one above the other by the various floods of the rivers. . . In every concavity at the summit of the mountains we shall always find the divisions of strata in the rocks.”
Leonardo appears to have grasped the law of superposition, which would later be articulated fully by the Danish scientist Nicolaus Steno in 1669: in any sequence of sedimentary rocks, the oldest rocks are those at the base. He also appears to have noticed that distinct layers of rocks and fossils could be traced over long distances, and that these layers were formed at different times:
“. . . the shells in Lombardy are at four levels, and thus it is everywhere, having been made at various times.”
Nearly three hundred years later, the rediscovery and elaboration of these principles would make possible modern stratigraphy and geological mapping.
Leonardo understood that marine fossils found in the mountains around Tuscany were the remains of ancient life forms, and that they provided evidence that ancient seas once covered areas that are now dry land. In Leonardo’s day there were at least two main hypotheses of how it was that shells and other living creatures were found in rocks on the tops of mountans. Some believed the shells to have been carried there by the Biblical Flood; others thought that these shells had grown in the rocks. Leonardo had no patience with either hypothesis, and refuted both using his careful observations. Concerning the second hypothesis of shells growing in rocks, he scathingly wrote:
“such an opinion cannot exist in a brain of much reason; because here are the years of their growth, numbered on their shells, and there are large and small ones to be seen which could not have grown without food, and could not have fed without motion — and here they could not move.”
Leonardo also refused to believe the prevailing view that fossils were simply carried to their present destinations by the biblical deluge and he suspected a much older earth than the Bible described.
“As to those who say that shells existed for a long time and were born at a distance from the sea, from the nature of the place and of the cycles, which can influence a place to produce such creatures — to them it may be answered: such an influence could not place the animals all on one line, except those of the same sort and age; and not the old with the young, nor some with an operculum and others without their operculum, nor some broken and others whole, nor some filled with sea-sand and large and small fragments of other shells inside the whole shells which remained open; nor the claws of crabs without the rest of their bodies . . .”(ref)
Indeed, Leonardo doubted the very existence of a single worldwide flood, noting that there would have been no place for the water to go when it receded. He also noted:
“if the shells had been carried by the muddy deluge they would have been mixed up, and separated from each other amidst the mud, and not in regular steps and layers — as we see them now in our time.”
He noted that rain falling on mountains rushed downhill, not uphill, and suggested that any Great Flood would have carried fossils away from the land, not towards it. He described sessile fossils such as oysters and corals, and considered it impossible that one flood could have carried them 300 miles inland, or that they could have crawled 300 miles in the forty days and nights of the Biblical flood.
So, how did those shells come to lie at the tops of mountains? Leonardo’s answer was remarkably close to the modern one: fossils were once-living organisms that had been buried at a time before the mountains were raised:
“it must be presumed that in those places there were sea coasts, where all the shells were thrown up, broken, and divided. . .”
Where there is now land, there was once ocean. It was possible, Leonardo thought, that some fossils were buried by floods — this idea probably came from his observations of the floods of the Arno River and other rivers of north Italy — but these floods had been repeated, local catastrophes, not a single Great Flood. To Leonardo da Vinci, as to modern paleontologists, fossils indicated the history of the Earth the scale of which extends far beyond human records. As Leonardo himself wrote:
“Since things are much more ancient than letters, it is no marvel if, in our day, no records exist of these seas having covered so many countries. . . But sufficient for us is the testimony of things created in the salt waters, and found again in high mountains far from the seas.”
Obvious as this may be today, in 1508, recognizing fossils as the remains of once living creatures was not a view shared by a majority and in fact in direct opposition to the prevailing scientific thought. As late as 1691 British biologist John Ray denied such an idea.
Comparative Anatomy: Leonardo da Vinci started his observations on the anatomy of the human body at the time he was apprenticed to Andrea del Verrocchio, who insisted that all his pupils learn anatomy. As Leonardo da Vinci became successful as an artist, he was given permission to dissect human corpses at the hospital Santa Maria Nuova in Florence. Later da Vinci dissected in Milano at the hospital Maggiore and in Rome at the hospital Santo Spirito (the first mainland Italian hospital). From 1510 to 1511 Leonardo da Vinci collaborated with the doctor Marcantonio della Torre (1481 to 1511). In 30 years, Leonardo da Vinci dissected 30 male and female corpses of different ages. Together with Marcantonio, da Vinci prepared to publish a theoretical work on anatomy and made more than 200 drawings. From this experience of anatomy and his observations of animals, Leonardo noticed the striking similarities in form and structure between humans and other animals:
an easy matter to men to acquire universality, for all
Much later a scientific study of comparative anatomy and embryology to understand evolution was carried out by Carl Gegenbaur, who died over a century ago in 1903. Gegenbaur a senior colleage of Ernst Haeckel put forth his views in his book Grundzüge der vergleichenden Anatomie (1859; Elements of Comparative Anatomy) which became the standard textbook, at the time, of evolutionary morphology, emphasizing that structural similarities among various animals provide clues to their evolutionary history. Carl Gegenbaur noted that the most reliable clue to evolutionary history is homology, the comparison of anatomical parts which have a common evolutionary origin.
This article is based in part on Fritjof Capra’s excellent new synthesis, The Science of Leonardo: Inside the Mind of the Great Genius of the Renaissance.
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