Early theories Philosophers began to think about how animals acquired form in the
womb in
classical antiquity.
Aristotle asserts in his
Physics treatise that according to
Empedocles, order "spontaneously" appears in the developing embryo. In his
The Parts of Animals treatise, he argues that Empedocles' theory was wrong. In Aristotle's account, Empedocles stated that the
vertebral column is divided into vertebrae because, as it happens, the embryo twists about and snaps the column into pieces. Aristotle argues instead that the process has a predefined goal: that the "seed" that develops into the embryo began with an inbuilt "potential" to become specific body parts, such as vertebrae. Further, each sort of animal gives rise to animals of its own kind: humans only have human babies.
Recapitulation , who argued for
recapitulation of evolutionary development in the embryo, and
Karl Ernst von Baer's
epigenesis A
recapitulation theory of evolutionary development was proposed by
Étienne Serres in 1824–26, echoing the 1808 ideas of
Johann Friedrich Meckel. They argued that the embryos of 'higher' animals went through or recapitulated a series of stages, each of which resembled an animal lower down the
great chain of being. For example, the brain of a human embryo looked first like that of a
fish, then in turn like that of a
reptile,
bird, and
mammal before becoming clearly
human. The embryologist
Karl Ernst von Baer opposed this, arguing in 1828 that there was no linear sequence as in the great chain of being, based on a single
body plan, but a process of
epigenesis in which structures differentiate. Von Baer instead recognized four distinct animal
body plans: radiate, like
starfish; molluscan, like
clams; articulate, like
lobsters; and vertebrate, like fish. Zoologists then largely abandoned recapitulation, though
Ernst Haeckel revived it in 1866.
Evolutionary morphology (a chordate), B. Larval
tunicate, C. Adult tunicate.
Kowalevsky saw that the
notochord (1) and gill slits (5) are shared by tunicates and vertebrates. From the early 19th century through most of the 20th century,
embryology faced a mystery. Animals were seen to develop into adults of widely differing
body plan, often through similar stages, from the egg, but zoologists knew almost nothing about how
embryonic development was controlled at the
molecular level, and therefore equally little about how
developmental processes had evolved.
Charles Darwin argued that a shared embryonic structure implied a common ancestor. For example, Darwin cited in his 1859 book
On the Origin of Species the
shrimp-like
larva of the
barnacle, whose
sessile adults looked nothing like other
arthropods;
Linnaeus and
Cuvier had classified them as
molluscs. Darwin also noted
Alexander Kowalevsky's finding that the
tunicate, too, was not a mollusc, but in its larval stage had a
notochord and pharyngeal slits which developed from the same germ layers as the equivalent structures in
vertebrates, and should therefore be grouped with them as
chordates. 19th century zoology thus converted
embryology into an evolutionary science, connecting
phylogeny with
homologies between the germ layers of embryos. Zoologists including
Fritz Müller proposed the use of embryology to discover
phylogenetic relationships between taxa. Müller demonstrated that
crustaceans shared the
Nauplius larva, identifying several parasitic species that had not been recognized as crustaceans. Müller also recognized that
natural selection must act on larvae, just as it does on adults, giving the lie to recapitulation, which would require larval forms to be shielded from natural selection. , may arise, without molecular evidence. But without molecular evidence, progress stalled. He modelled catalysed chemical reactions using
partial differential equations, showing that patterns emerged when the chemical reaction produced both a
catalyst (A) and an
inhibitor (B) that slowed down production of A. If A and B then diffused at different rates, A dominated in some places, and B in others. The Russian biochemist
Boris Belousov had run experiments with similar results, but was unable to publish them because scientists thought at that time that creating visible order violated the
second law of thermodynamics.
The modern synthesis of the early 20th century In the so-called
modern synthesis of the early 20th century, between 1918 and 1930
Ronald Fisher brought together Darwin's theory of
evolution, with its insistence on natural selection,
heredity, and
variation, and
Gregor Mendel's
laws of genetics into a coherent structure for
evolutionary biology. Biologists assumed that an organism was a straightforward reflection of its component genes: the genes coded for proteins, which built the organism's body. Biochemical pathways (and, they supposed, new species) evolved through
mutations in these genes. It was a simple, clear and nearly comprehensive picture: but it did not explain embryology.
Sean B. Carroll has commented that had evo-devo's insights been available, embryology would certainly have played a central role in the synthesis. by showing that evolution could occur by
heterochrony, such as in
the retention of juvenile features in the adult. However, despite de Beer, the modern synthesis largely ignored embryonic development to explain the form of organisms, since population genetics appeared to be an adequate explanation of how forms evolved.
The lac operon . Top: repressed. Bottom: active. (1)
RNA Polymerase, (2)
Repressor, (3)
Promoter, (4) Operator, (5)
Lactose, (6–8)
protein-encoding genes, controlled by the switch, that cause lactose to be digested. In 1961,
Jacques Monod,
Jean-Pierre Changeux and
François Jacob discovered the
lac operon in the
bacterium Escherichia coli. It was a cluster of
genes, arranged in a feedback
control loop so that its products would only be made when "switched on" by an environmental stimulus. One of these products was
an enzyme that splits a sugar, lactose; and
lactose itself was the stimulus that switched the genes on. This was a revelation, as it showed for the first time that genes, even in organisms as small as a bacterium, are subject to precise control. The implication was that many other genes were also elaborately regulated.
The birth of evo-devo and a second synthesis In 1977, a revolution in thinking about evolution and developmental biology began, with the arrival of
recombinant DNA technology in
genetics, the book
Ontogeny and Phylogeny by
Stephen J. Gould and the paper
"Evolution and Tinkering" by
François Jacob. Gould laid to rest Haeckel's interpretation of evolutionary embryology, while Jacob set out an alternative theory. at last including embryology as well as
molecular genetics, phylogeny, and evolutionary biology to form evo-devo. In 1978,
Edward B. Lewis discovered
homeotic genes that regulate embryonic development in
Drosophila fruit flies, which like all insects are
arthropods, one of the major
phyla of invertebrate animals.
Bill McGinnis quickly discovered homeotic gene sequences,
homeoboxes, in animals in other phyla, in
vertebrates such as
frogs,
birds, and
mammals; they were later also found in
fungi such as
yeasts, and in
plants. There were evidently strong similarities in the genes that controlled development across all the
eukaryotes. In 1980,
Christiane Nüsslein-Volhard and
Eric Wieschaus described
gap genes which help to create the segmentation pattern in
fruit fly embryos; they and Lewis won a
Nobel Prize for their work in 1995. Later, more specific similarities were discovered: for example, the
distal-less gene was found in 1989 to be involved in the development of appendages or limbs in fruit flies, the fins of fish, the wings of chickens, the
parapodia of marine
annelid worms, the ampullae and siphons of tunicates, and the
tube feet of
sea urchins. It was evident that the gene must be ancient, dating back to the
last common ancestor of bilateral animals (before the
Ediacaran Period, which began some 635 million years ago). Evo-devo had started to uncover the ways that all animal bodies were built during development. ==The control of body structure==