These pages have been left in this location as a service to the numerous websites around the world which link to this content. The original authors are no longer at the University of Leeds, and the former Centre for Human Biology became the School of Biomedical Sciences which is now part of the Faculty of Biological Sciences.
Nielson C & Nørrevang A (1985) The trochlea
theory: an example of life cycle Phylogeny In The origin
and relationship of Lower Invertebrate Groups. Ed Conway Morris
et al. pp28-41. Oxford University Press, Oxford.(OVERHEAD)
So, where are we?
The 34 types or Bauplans that we have today are a subset,
possibly a random subset, of at least twice that number of Bauplans
that seem to have been generated fairly rapidly after the origin
of the metazoa. We can reasonably suppose that these did not
arise from 70 or so events leading to the formation of 70 different
metazoan types. It is most likely that most of these (except
possibly the sponges) were the product of one transition from
single celled animal to multicelled animal. It is likely that
if the metazoans arose only once there is some sort of relationship
between extinct and extant phyla. It is also likely that the
different phyla arose due to modifications of a new property
of metazoans - their embryology - as exemplified by their larval
So the important questions are: (OVERHEAD)
1. How are phyla related?
2. How are phyla (and the lesser divisions down to species) converted i.e. how is one changed into another?
3. Evolution and development - Is there a link?
If we can answer these we can also answer some other difficult questions a bit nearer home: (OVERHEAD)
1. How does an egg turn into an insect/frog/mouse/man?
2. Why does a particular sort of egg always become a frog?
3. What is the difference between a frog's egg and a hen's?
or What are the similarities between a frog's egg and
We look at relationships between animals in terms of similarities.
We have already said that the most similar animals are identical
twins, then brother and sister etc. We were, strictly speaking,
talking about genes, but the effects of genes, phenotype
can be substituted. Identical twins are not recognised by their
karyotypes but by their appearance. Two cows are probably similar
if they look alike. A cow is more like a sheep than it is like
a fish. So phenotypic appearance is important i.e. animals are
classified by their morphology.
Similarities between extant and extinct creatures are more
difficult: we are unlikely to see a modern human foot on a fossil
human ancestor: but it is still recognisably a foot, and so are
the feet of dinosaurs. The foot is an homologous structure i.e.
the same, probably descended from a common ancestral foot. We
have said that the wing of a bird is not homologous with the
wing of an insect. These do the same thing, but not in the same
way: they are analogous. Often a similar feature arises more
than once: the wing of a bat is not descended from the wing of
a bird: are they analogous? No. Are they homologous No, not quite
because one did not descend from the other. They are examples
of convergent evolution, nearly homologous because each is homologous
with the forelimb of some non-flying ancestor.
Sometimes homologous structures are so modified that it is
difficult to see the similarity. What do we do then? When talking
about different animal groups I gave you a clue: some animals
have similar larvae, i.e. similar embryonic stages.
We now reach a point where we have to define terms. First
of all lets look at evolution and development.
In fact these terms are quite mixed up in laymen's language.
Going to the O.E.D. we find:
1. opening out (of roll, bud etc.), appearance (of events etc.) in due succession
2. development (of organism, human society, the Universe, design, argument etc.): origination of species by development from earlier forms, not by special creation.
(of organism, design, argument etc.) Theory of E (that the
embryo is not created by fecundation, but developed from a pre-existing
form); origination of species by development from earliest forms
So evolution, in non scientific terms, means both the development
of an individual and the development of a species. And development
covers evolution too. Are these two the same thing? If not how
are they related? Why are they considered to be related at all?
Well, earlier, pre-evolutionary views certainly fit well with
this idea of evolution as unfolding, like a scroll. There was
a widespread view that all human bodies were created fully formed
but rolled up, as it were, in the ovaries of Eve. A complementary
theory described the homunculus, perfectly formed, rolled up
in every sperm. (OVERHEAD)
The other view of evolution, as progressive change through time is usually attributed to Darwin, but he, interestingly, only uses the word once in the Origin of the Species - then it is the last word in the book
There is a grandeur ...evolved (OVERHEAD)
Elsewhere he speaks of descent with modification.
There are perfectly good reasons for distinguishing the two
developments, or the two evolutions, one of the organism the
other of the species. For instance the eggs of many (but not
all) animals are separated into soma and germa. The
soma develops into the body of the individual: the germa
goes on to form the gametes, future generations.
Modifications in the soma (somatic mutation, teratogens) will
not affect the species but will affect the operation of the genetic
program during an animal's lifetime: mutations in the germa will
not influence the development of the current generation but may
influence the future of the species.
These two processes have, of course long been distinguished.
The development of an individual from fertilisation to maturity
is ontogeny: the development of a species or lineage is
The idea of a relationship between phylogeny and ontogeny
is an old one, but was probably crystallised by looking at embryos
of different species. These all look suspiciously alike, and
the younger the embryos the more similar they look.
This was first noticed by Meckel (1781-1833) who studied human
embryos and saw them passing through what he considered to be
a hierarchy of forms: fish, reptile, mammal, human. Coincidentally
this was the same order that the fossils that were being discovered
at the same time occupied in geological strata. Meckel very clearly
saw that the same laws covered both 'development' of the individual
and 'evolution' of the species. By evolution, of course he meant
unfolding - he was pre-Darwinian. Did the developing embryo recapitulate
the past of its species? Did it climb up its own family tree?
Von Baer (1792-1876), another good comparative embryologist,
took over from there. His problem was that he found structures
in higher animals - such as the yolk sac in birds - that were
not present in lower animals. Also birds did not, at any stage,
have fish tails. Therefore recapitulation as such was out. What
was happening was a gradual specialisation: all vertebrate embryos
first developed characteristics of the phylum vertebrata, then
fishes developed characteristics of the class Pisces, birds of
the class Aves, rabbits of the class Mammalia etc. A little later
they developed more specific details, like fins or floppy ears.
Now we have a problem. Do the Phyla, which we have
defined by going from more similar to less similar have fundamental
similarities as well as differences? Von Baer would expect similarity
in cow embryos, and in mammalian embryos, but how about an earthworm
embryo and a sea urchin. Are they similar?
We have already looked at the ideas of Nielson He noticed
similarity between the ciliary feeding structures of adult Rotifers
and several types of Spiralian (animals with spiral cleavage,
see later) and other protostomian larvae, and the difference
between these and deuterostomes. (OVERHEAD). Essentially,
remember, protostomes and deuterostomes differ in collecting
food upstream or downstream of their cilia. Some years it was
proposed that animals might be related via different types of
free living, planktonic ancestors, three of which gave rise to
creeping descendants of various types.
The early part of this is nothing new: in fact it corresponds
with the gastraea theory of Haeckel (1834-1919) which we talked
about last week. Haekel reasoned, remember, that all multicellular
organisms arose from unicellular organisms, and that this probably
happened only once. The different bodyplans of the different
phyla arose from a hypothetical, archaetypical Blastaea a
hollow ball of cells. We have already talked about this, and
said it had to be an animal not a plant, because further progress
depends on the fact that not all cells feed - meaningless in
plants - and have connections: we identified a likely candidate
in the choanoflagellate.
Once settled on the bottom, the Gastraea was also seen
by Haekel to be a common ancestral type, although there are gastraea
Not everyone agrees with this: (OVERHEAD)
A giddy little Gastrula, gyrating round and round,
Was thought to show the way we get our enteron profound:
A little whirlpool in its wake maintained by a tasty store,
A pocket sank to lodge it all and left a blastopore.
Invagination surely is a thing of later date-
Procedure speeded up to suit the embryonic state:
The cells as loose irregulars build up the lower grades,
And yield but slowly, step by step, to organised brigades.
Walter Garstang, Larval Forms, Blackwell, Oxford 1951
(OVERHEAD) Not all animals will conform to this scheme,
but most do.
It has been argued that small eggs and pelagic larvae are
nowadays a feature only of rather large marine animals which
produce large numbers of eggs. Small animals produce few eggs
and protect their brood. But ancestral forms are likely to be
small: increase in size is a well known evolutionary trend. (Known
as Cope's law or Cope's rule - but is it true? Some of you are
writing an essay about this.). Therefore (small) ancestors are
not likely to have had pelagic larvae. This, of course, applies
now, but not necessarily 600mya. when there were presumably less
multicellular predators in the plankton.
We should perhaps look a little more closely at the concept
of a larval form. We are quite accustomed to organisms which
change their proportions as they grow (kittens, puppies, children
OVERHEAD,OVERHEAD - the phenomenon of allometry) but perhaps
not so used to the idea of specific adaptations at different
stages of life (OVERHEAD,OVERHEAD) perhaps because we
do not perceive that mammals have them, although insects and
many other animals do.. We do, in fact, have them: but the non
self-feeding part of our life cycle, the part where we live in
an aquatic environment is passed either in mum. There is in fact
no reason why any stage of a life cycle should not develop specialisations
and, say feed in a different way or live in a different place.
There is also no reason why changes in lifestyle should not be
accompanied by changes in shape - metamorphoses. And there is
really no difference in kind between a metamorphosis, which occurs
suddenly and maturation which occurs rather more slowly, except
the time scale. There is also no reason why development shouldn't
get out of step with itself to such an extent that one of these
juvenile forms should not develop gonads and breed. If that happens,
as with the axolotl, a Mexican salamander, the 'adult' stage
can be missed out altogether.
This suggests that there must be mechanisms which can change
the relative rates of growth during embryology: the next question
is what are they?
So we have a working hypothesis on classification which derives
three sorts of animalia from larval forms swimming in the sea.
Haeckel required that new evolutionary stages be added to
the end of the life cycle of ancestral forms. As well as
a phylogenetic tree we have also described embryology: the blastula,
the gastrula are the same as the blastaea and the gastraea. Abolition
of free swimming larval forms (brood protection again) could
generate an embryology which resembles our phylogenetic tree,
but in which some stages are not free living, so don't really
need to work and could be compressed or not finished - in this
way some features might become rudimentary like the appendix
or the tail in man.
Von Baer extended his studies by demonstrating that all vertebrate
embryos had three germ layers and that the outer and inner ones
corresponded to the two layers of coelenterates, and Lankester
extended the triple layer of cells to all animal phyla except
the single celled animals and the two layered coelenterates.
So, the embryo is the ancestor, and all phyla are related.
This page is maintained by Steve Paxton