6.2 Body Plan
87
that could have been intermixed originally. As Stephen Jay Gould (1989) formulates
it, the removal of most groups by extinction leaves large morphological gaps among
the survivors. Further diversification can be triggered by environmental innovations,
like the emergence of new genera after great extinctions or new operating systems
when smartphones burst in.
The three tissue layers of the new kind of triploblastic animals are already formed
in the embryo. The outer layer forms skin, while the middle one forms muscles and
all internal organs, except the digestive tract, which develops from the inner layer.
This body plan is absent in primitive animals originating prior to the Cambrian, like
sponges and medusas. In most cases, it includes a body cavity (coelom) containing
the internal organs. Another innovative feature is bilateral symmetry, distinguishing
between the top and bottom sides, and distinct front and back ends (Fig. 6.4, right).
This arrangement looks to us very natural, perhaps because we are like this ourselves, and so are our cars, and even our dwellings, both family homes and palaces,
with the main entrance and large windows at the front and garbage removed at the
back. Both in cars and in homes, the external walls and the interior are built from different materials and decorated in a different way, and they hold us, as their essential
internal organs, in their cavities. Of course, when moving, it is good to look ahead,
but this is not obviously necessary for houses, and, indeed, sophisticated architects
may design centrally symmetric forms. Echinoderms (e.g., starfish) abandoned the
bilateral symmetry of their ancestors after adopting a sedentary lifestyle – but it was
retained by their larvae.
Here we come to something that appears strange and even wasteful. Our forms
develop directly from the embryo gradually forming in the womb. Recognizably human features do not come in right away. In the early 19th century, Johann Friedrich
Meckel and ´
Etienne Serres drew attention to the similarity between all early embryonal forms, repeating the course of evolution. Ernst Haeckel (1874) summed it up
in the phrase “ontogeny recapitulates phylogeny”, as illustrated by Fig. 6.5. But the
development of many animals is not as straightforward.
Earlier forms can be observed directly in animals hatched from laid eggs. Even
among vertebrates, tadpoles look and live like fish before turning into adult amphibians. But the metamorphosis from a juvenile to an adult form can be still more
radical. The novelist Anatole France once observed that he would design humans as
butterflies, so that they would first work as sexless wormlike caterpillars and then,
after acquiring the necessary means, enjoy freedom and love – not like us, suffering
first from efforts and anxieties and then from infirmities and sickness. But would
you enjoy the state of a pupa, suspended lifeless while gradually shedding the forms
of your youth and re-emerging as a strange colorful creature?
Many echinoderm and arthropod species go through a larval stage, and some
have even two or more distinct larval phases. This is the case of indirect development: a package of cells that are destined to grow into an adult form is set aside,
while the embryo develops into a larva with a different body plan. Freeman Dyson
(2004) describes the advantages of this arrangement, not unlike the vision of Anatole France: the embryo provides life support to the adult, [. . . ] whereas the adult is
free to evolve elaborate and fine-tuned structures. Davidson et al (1995) argued that
87
that could have been intermixed originally. As Stephen Jay Gould (1989) formulates
it, the removal of most groups by extinction leaves large morphological gaps among
the survivors. Further diversification can be triggered by environmental innovations,
like the emergence of new genera after great extinctions or new operating systems
when smartphones burst in.
The three tissue layers of the new kind of triploblastic animals are already formed
in the embryo. The outer layer forms skin, while the middle one forms muscles and
all internal organs, except the digestive tract, which develops from the inner layer.
This body plan is absent in primitive animals originating prior to the Cambrian, like
sponges and medusas. In most cases, it includes a body cavity (coelom) containing
the internal organs. Another innovative feature is bilateral symmetry, distinguishing
between the top and bottom sides, and distinct front and back ends (Fig. 6.4, right).
This arrangement looks to us very natural, perhaps because we are like this ourselves, and so are our cars, and even our dwellings, both family homes and palaces,
with the main entrance and large windows at the front and garbage removed at the
back. Both in cars and in homes, the external walls and the interior are built from different materials and decorated in a different way, and they hold us, as their essential
internal organs, in their cavities. Of course, when moving, it is good to look ahead,
but this is not obviously necessary for houses, and, indeed, sophisticated architects
may design centrally symmetric forms. Echinoderms (e.g., starfish) abandoned the
bilateral symmetry of their ancestors after adopting a sedentary lifestyle – but it was
retained by their larvae.
Here we come to something that appears strange and even wasteful. Our forms
develop directly from the embryo gradually forming in the womb. Recognizably human features do not come in right away. In the early 19th century, Johann Friedrich
Meckel and ´
Etienne Serres drew attention to the similarity between all early embryonal forms, repeating the course of evolution. Ernst Haeckel (1874) summed it up
in the phrase “ontogeny recapitulates phylogeny”, as illustrated by Fig. 6.5. But the
development of many animals is not as straightforward.
Earlier forms can be observed directly in animals hatched from laid eggs. Even
among vertebrates, tadpoles look and live like fish before turning into adult amphibians. But the metamorphosis from a juvenile to an adult form can be still more
radical. The novelist Anatole France once observed that he would design humans as
butterflies, so that they would first work as sexless wormlike caterpillars and then,
after acquiring the necessary means, enjoy freedom and love – not like us, suffering
first from efforts and anxieties and then from infirmities and sickness. But would
you enjoy the state of a pupa, suspended lifeless while gradually shedding the forms
of your youth and re-emerging as a strange colorful creature?
Many echinoderm and arthropod species go through a larval stage, and some
have even two or more distinct larval phases. This is the case of indirect development: a package of cells that are destined to grow into an adult form is set aside,
while the embryo develops into a larva with a different body plan. Freeman Dyson
(2004) describes the advantages of this arrangement, not unlike the vision of Anatole France: the embryo provides life support to the adult, [. . . ] whereas the adult is
free to evolve elaborate and fine-tuned structures. Davidson et al (1995) argued that