Ahead of his time is putting it
moderately for Charles Darwin. The father of evolution had conjectures that were only proved, or
greatly substantiated, decades after his death in 1882, in some cases not until
recently. Today, evidence that unequivocally supports his theory of evolution
by natural selection, as well as other surmises he had, comes from an array of
scientific disciplines, including paleontology, geology, biochemistry,
genetics, molecular biology, and, most recently, evolutionary developmental
biology, or "evo devo." "The notion that all these lines of
evidence could converge and give a common answer to the question of where we
came from is truly powerful," says Brown University biologist Kenneth
Miller. "This is the reason why scientific support for the theory of
evolution is so overwhelming." Here, bone up on Darwin's most
prophetic ideas.—Peter Tyson
Darwin wasn't the first to
suggest the theory of evolution—that all living things originate from
ancestral forms and that their distinguishable differences are due to
modifications in successive generations. But no one gave it more thought, or
provided more evidence for it, or more deeply ingrained the theory into our
collective consciousness than Charles Darwin. Today, researchers using the
genetic techniques of "evo devo" can trace the evolution of the
various kinds of organisms that Darwin fit into his "tree of life"
(here, from an 1837 notebook, his first such tree). As the geneticist Theodosius
Dobzhansky once wrote, "Nothing in biology makes sense except in the
light of evolution."
Evolution happens through natural
Darwin was not content to say that
organisms evolve. The theory of evolution, he wrote, "even if well
founded, would be unsatisfactory until it could be shown how the innumerable species inhabiting this world have
been modified so as to acquire that perfection of structure and coadaptation
that most justly excites our admiration" [italics added]. In what has
been called "the greatest idea anyone ever had," Darwin theorized
how those modifications occurred—through a process he called natural selection,
which holds that individuals within a species that are best suited to their
environment will survive and reproduce more than those less suited, thereby
passing on their more useful traits and genetic qualities to successive
generations. "Nothing that we have learned in the intervening 175
years," writes paleontologist Niles Eldredge of the time since Darwin
first started formulating his theory, "has contravened Darwin's
basic description of how natural selection works."
Evolution happens through
natural selection. (continued)
Suitably, one of the most striking
examples of natural selection in action concerns the very Galapagos finches
that Darwin made famous. Since 1973, biologists Peter and Rosemary Grant,
working on the tiny island of Daphne Major in the Galapagos, have studied a
species of finch called Geospiza fortis
(upper right in illustration, which appeared in Darwin's 1839 book about
his five-year journey aboard the Beagle). After a drought in 1977 devastated plants bearing small seeds, more
than 1,000 of the 1,200 G. fortis
finches on the island died. The Grants discovered that larger G.
fortis, which could break open larger seeds
than smaller G. fortis could,
survived better. The survivors mated in 1978, and, on average, their offspring
had beaks 4 percent larger than those of the previous generation. Following
another drought in 2003, G. fortis
with smaller beaks survived better, in part because of stiff competition for
bigger seeds after a larger finch species, G. magnirostris, settled the island. Between 2003 and 2005, the
Grants found, G. fortis beaks
shrank by 5 percent.
Evolution by natural selection
must have a mechanism.
Darwin knew in his heart that
evolution by natural selection happened, but he didn't know how. In fact, he proposed a mechanism of heredity that
proved to be dead wrong, something he called pangenesis. Ironically, the actual
mechanism of heredity—genetics—was revealed in Darwin's
lifetime, though he never knew it. Working with pea plants, the Austrian priest
Gregor Mendel discovered the fundamental laws of genetics in 1865. But
Mendel's work was neglected until 1900, and it wasn't until the
1940s that scientists identified DNA as the genetic material (here, Francis
Crick's first sketch of DNA's double helix structure). Today, the
clear focus of the revolutions now under way in genetics, medicine, and other
fields is on the genomes of living organisms.
The mechanism—whatever it
is—is natural, not supernatural.
Darwin held off on publishing his
theory for 20 years, until forced to by fellow naturalist Alfred Russel
Wallace, who had independently come up with the same idea. One of the reasons
for Darwin's delay was he knew what a shock his theory would represent to
Christians, including his beloved wife Emma, who believed that God had created
all living things, whole and independent of all others. Darwin didn't
hold this view. His friend, the biologist T. H. Huxley, agreed. "As a
natural process of the same character as the development of a tree from its
seed, or of a fowl from its egg," Huxley wrote, "evolution excludes
creation and all other kinds of supernatural intervention." Of course,
many people around the world still believe in a divine instrument of creation.
Embryology is "the
strongest single class of facts in favor of change of forms."
Darwin considered the process that
a fertilized egg goes through on the way to becoming a multibillion-celled adult
as crucial to understanding how basic anatomical parts of both animals and
plants change over evolutionary time. The new science of evolutionary
developmental biology, "evo devo," is revealing the truth behind
Darwin's assertion quoted above. Until recently, scientists could say
that forms do change, but they couldn't say exactly how, because they didn't know which genes were
essential for the development of any organism. They now know that, remarkable
as it may seem, all organisms on Earth, from microbes to man (a human embryo is
seen here), share a common "tool kit" of so-called master genes,
which govern how bodies and body parts form. When and which of these genes are
turned "on" or "off" during the embryonic stage
determines how each organism is built.
Sexual selection also drives
The peacock posed an enigma to
Darwin. How could its extravagant tail feathers have come about? How do they
aid in the "survival of the fittest" (a phrase coined by the
philosopher Herbert Spencer)? Darwin proposed an answer in his book Descent
of Man, in which he introduced a novel
concept—sexual selection, the critical role that the opposite sex plays
in the evolution of traits. Over time, Darwin believed, peahen choices about
which peacocks constituted the finest mates could have led to the males'
absurdly ostentatious tails. In the 1990s, biologist Marion Petrie of the
University of Newcastle-upon-Tyne showed Darwin's hunch to be true. On
average, peacocks have 150 "eyes" on their tails. By clipping off
just a few of those eyes, Petrie found he could significantly reduce a
male's chances of mating; males with fewer than 130 eyes rarely mated at
all. Petrie reasoned that the healthier a peacock was, the more he could invest
in the production of eyes—something that peahens clearly saw for
All animals, including humans,
descend from a common ancestor.
Darwin knew the idea that man was
just another animal would go over like a lead balloon in Victorian society. But
in the Origin of Species, he was
unequivocal on this notion, writing that "I should infer from analogy
that probably all the organic beings which have ever lived on this earth have
descended from one primordial form, into which life was first breathed."
Evidence supporting this view has accumulated ever since, with arguably the
strongest support coming from evo devo. Scientists comparing the genomes of
members of the three domains of life—archaea, bacteria, and the eukaryotes
(animals, plants, fungi, and protists)—have discovered about 500 genes
that all living things share. These "immortal" genes have survived
essentially unchanged for over two billion years (including the portion of a
protein's sequence seen here, with shared amino acids indicated by shading). Says
Brown's Ken Miller: "Despite the extraordinary diversity of life,
all living organisms share a nearly identical set of essential genes,
reflecting their evolutionary development from a common ancestor."
Humans evolved from an ape-like
Fearing that many of his contemporaries would be deeply offended if he suggested that humans and apes share
a common ancestor, Darwin barely touched on human evolution in the 1859 Origin
of Species. But by the 1871 Descent
of Man, he was ready, stating baldly that
"man is descended from a hairy, tailed quadruped, probably arboreal in
its habits." He also maintained that the difference between the mind of
man and that of a chimpanzee or gorilla is a matter of degree, not of kind.
Summing up, Darwin wrote, "We must … acknowledge, as it seems to
me, that man with all his noble qualities … still bears in his bodily
frame the indelible stamp of his lowly origin." Today, many a schoolchild
can cite the figure perhaps most often called forth in support of this
view—namely, that we share almost 99 percent of our DNA with our closest
living relative, the chimpanzee. (The illustration shown here appeared in T. H.
Huxley's 1863 book Man's Place in Nature. All skeletons are to scale except the gibbon, which is shown
twice natural size.)
Modern humans arose in Africa.
In Darwin's day, virtually no
fossils of early humans were known—one of the few was Neanderthal Man, discovered
in Germany's Neander Valley in 1856—nor could anyone date them
reliably. But noticing our close anatomical relationship to chimps and
gorillas, Darwin surmised that Homo sapiens had evolved in Africa. In his customary careful way, he wrote,
"It is somewhat more probable that our early progenitors lived on the
African continent than elsewhere." Today, the fossil known as Lucy is
only the most famous of numerous fossils of human and human-like species whose
age and other particulars confirm the notion that modern humans first came into existence in Africa. Here, a
1999 evolutionary tree based on DNA shows how the five great apes, including us, arose from a
common ancestor. Branch length indicates how far the genes of each population
have diverged from those of near relatives.
The Earth is at least several
hundred million years old.
In 1658, based on his reading of
the Bible and on historical records, the Irish clergyman James Ussher declared
that God had created the Earth on October 22, 4004 B.C. Geologists of
Darwin's day begged to differ, arguing for a far older Earth. Darwin,
knowing that evolution required vastly more time than just six millennia,
concurred. In the first edition of the Origin of Species, he estimated our planet to be at least several
hundred million years old. In fact, as is well known today, it is far more
primeval. In the 1900s, after Marie and Pierre Curie discovered radioactivity,
physicists learned how to use the half-life of uranium and other radioactive
elements to estimate the age of rocks. The oldest minerals found on Earth are
about 4.1 to 4.2 billion years old, indicating our planet is at least that old.
But meteorites, which scientists believe formed at the same time as the planets
and which, unlike the Earth, are not geologically active and therefore retain
evidence of their formation, date even further back. Using several different
dating methods on dozens of different kinds of meteorites, scientists have, in
instance after instance, come up with an age of roughly 4.5 billion years—the
current widely accepted estimate of our planet's age.
Gaps in the fossil record will
be filled in with key transitional fossils.
In his notebooks, Darwin worried
about the many gaps in the fossil record that then existed, "like pages
torn from a book." Perhaps the most glaring was the complete lack of
fossils before the Cambrian Period (544-510 million years ago). "To the
question why do we not find rich fossiliferous deposits belonging to these assumed
earliest periods before the Cambrian system, I can give no satisfactory
answer," he wrote. If his theory of evolution was right, he knew such
gaps would eventually be filled in. Only in the past half century have
paleontologists unearthed fossils from the Precambrian, and it's a huge
and ever-growing trove, with the oldest fossils dating to about 3.5 billion
years ago. Moreover, they continue to unearth key fossils that bridge those
gaps Darwin bemoaned, including Archaeopteryx, shown here. To see a range of such transitional fossils, go to
An insect with a foot-long
tongue must exist to pollinate this orchid.
Not all of Darwin's
conjectures were so broad in scope or so earth-shaking; some were simple
predictions. Take the case of this species of orchid, Angraecum
sesquipedale, from Madagascar. When Darwin
saw this orchid in 1862, he, like anyone who saw it, was astonished by the
length of its spur, which can reach over a foot in length. (See long, slim tube in photo.)
"Astounding," he wrote. "What insect could suck it?"
For some as-yet unknown insect must, he insisted, and it had to have a
foot-long tongue to get at the plant's nectar, which pools at the very
base of the spur. Entomologists of his day were skeptical, for no such creature
had ever turned up. But several decades after Darwin's death in 1882,
scientists discovered a giant hawk moth in Madagascar, and it lapped the
orchid's nectar with, yes, a foot-long tongue. The moth was named Xanthopan
morganii praedicta in honor of his
prediction. Once again, Darwin was right.
Selected sources for this article
We recommend you visit the interactive version. The text to the left is provided for printing purposes.