March 26) Multicellular Animals: Who is who?


The evolutionary "big picture" for animals.

#1) Multicellular animals are classified into ~30 different phyla, all of which had already evolved by the Cambrian period, when the fossil record comes into focus.

Thus, you cannot trace evolutionary branching pathways of these phyla relative to each other, in the way that you CAN track evolutionary origins of gymnosperms or birds, etc. within phyla.

But the following general patterns of ancestry can be deduced:

#2) Sponges are the most primitive phylum of multicellular animals
But most experts now think that sponges are a side branch,
in the sense that the rest of multicellular animals are NOT descended from organisms like sponges.

#3) Sponges are believed to have evolved from a specific group of flagellated protozoa, called the "choanoflagellates" which also filter food out of water by means of a collar of cytoplasmic projections in a ring around single flagella.

#4) Researchers in the UNC Biology Department have made several important discoveries about sponges:

4A) Between 1905 and 1912, H. V. Wilson discovered that living sponge cells can survive being mechanically separated from one another by being squeezed through course cloth, and can then rearrange to form their correct functional anatomical patterns in a few days. Later Wilson showed that cells of soft corals (a more advanced phylum) can also sort out to re-form functioning individuals. (published in J. Experimental Zoology) Later, sorting out was demonstrated in embryos of sea urchins, frogs, birds and mice.

Wilson considered two alternative explanations for how the randomly arranged sponge cells were able to re-form anatomical structures.

(I) That the cells switch from being one differentiated cell type to another, according to their new positions; for example that cells became skin cells if they found themselves on the outside.

or (II) That differentiated cells crawl back to their correct locations; for example, that skin cells crawl back to the outer surface.

Wilson believed alternative #I was much more likely than #II, and spent the rest of his life, until the 1930s, trying to disprove #II. (nevertheless, hypothesis # II turned out to be correct)
{but textbooks gave Wilson credit for discovering it, anyway! But they often get him confused with E.B. Wilson of Columbia to whom he was NOT related, although they had been students together!!

Other researchers in Germany (Holtfreter- 1930s), and the USA (Trinkaus, Moscona, Steinberg - 1950s) proved that the true explanation is alternative #II, at least for vertebrate embryo cells.
For example, John P. Trinkaus & Peggy Groves used radioactively labeled thymidine to track which cells were accumulating where in chicken cells sorting out by differentiated cell type.

This research led to the discovery of cell-cell adhesion proteins that are very selective, so that liver cells stick better to other liver cells, brain cells stick to brain, uterus to placenta, etc.

Most embryologists now think that cell sorting (as first observed by Wilson) is caused entirely by cell-cell adhesion proteins. [presented as proven dogma on pages 385-386 of our textbook]

4B) In the 1980s, a UNC Biology graduate student, Cal Bond (now a Professor at Greensboro Univ.) working with me, discovered three more facts about sponges:
i) Sponges slowly crawl from place to place, instead of being sessile
ii) Sponge cells constantly rearrange all the time, just as actively as after dissociation
iii) The path of water flow through sponges is very different from what is shown in all the textbook diagrams. But we are still trying to prove this third one!

#5) Probably the jelly-fish/coral/Hydra/sea anemone phylum corresponds to the true evolutionary root of animals.

#6) Besides various minor side-branches, the two most successful side branches are the following

The branch leading to vertebrates and other chordates, and also including echinoderms

The branch leading to insects, spiders, crustaceans, etc. and other arthropods,
which also includes the phylum of molluscs (snails, clams, octopus etc. and the phylum of segmented worms (earthworms, leeches, etc.)

#7) Each invertebrate phylum represents a (surprisingly) different set of solutions to the biological problems: How to move?
How to breathe? How to eat? How to excrete wastes?
How to coordinate behavior? How to grow bigger? etc. etc.

Biology 105: Invertebrate Zoology has gotten several teaching awards
The actual differences between phyla are more extreme and interesting than in any science fiction stories. (& if you like that kind of science fiction, consider reading Olaf Stapledon's "Last and First Men", and "Starmaker" from the 1930s)

#8) Let's contrast arthropod anatomy with vertebrates

Exoskeleton: as contrasted to endoskeleton

An exoskeleton serves support and protection functions; but makes growth difficult (requires molting; shedding)

Specifically in insects, air reaches tissues through branching pipes from the surface.

Contrast with lungs & high pressure circulatory system in vertebrates.
(although many salamanders lack lungs, in case you are interested)

#9) Metamorphosis in insects, by molting = shedding & replacement of exoskeleton.

Contrast to metamorphosis of tadpoles to frogs or of sea squirt "tadpole" to sessile adult form.

Note the contrast between grasshopper
       pattern of partial metamorphosis

teensy grasshopper->tiny grasshopper->small grasshopper->bigger grasshopper

as compared with flies, butterflies, moths, ants, bees etc.
       complete metamorphosis
              Caterpillar ------> Butterfly

#10) Steps in evolutionary progress in vertebrates

Earliest vertebrate ancestors were filter-feeders; like sea squirts, "Amphioxus", and also larval lampreys

Jawless vertebrates that still exist today include lampreys and hagfish;
but at the beginning of the Cambrian ALL vertebrates were jawless.

The major evolutionary advances in vertebrates have been jaws, limbs, amniotic eggs
(= extraembryonic membranes?), and endothermy (warm bloodedness).

a) Jaws evolved in Cambrian, in a now-extinct kind of fish (evolved from supports for gills)

b) Limbs (from fins), arms and legs and bird wings.

c) Amniotic eggs (as in reptiles, birds, platypus)
Embryos have extraembryonic membranes, one of which is the amnion (the "bag of waters")

These four extraembryonic membranes form the placenta in mammals.

It has almost always been assumed that the extraembryonic membranes of reptiles evolved in egg-laying species, with several advantages, including keeping the embryo wet and storing food.

But many reptiles (e.g. copperheads), & many fish (guppies) are live-bearers, with embryos developing inside mother's body.

Is it possible that the 4 extraembryonic membranes (the amnion and the other 3 found in amniotic eggs) might have evolved first in live-bearing reptiles or amphibia

d) Warm-bloodedness (endothermy)

#11) Fossil record of human evolution goes back 4 million years.

Homo sapiens (our species) evolved ~ hundred thousand years ago.or more like 130,000 years ago (& probably evolved in Africa). We overlapped with Neanderthal Man (another species?) which didn't become extinct until ~ 30,000 years ago.

Chimpanzees are our closest living cousins; Gorillas are somewhat less closely related to us.

But the DNA base sequences of humans are less than two percent different than chimpanzees and gorillas!

#12) In Asia and especially North and South America, humans apparently caused the extinction of most large mammals, but much less so in Africa! Why?

Why is Africa the one continent where humans did NOT cause the extinction of most of the large mammals?

Could this be because humans themselves also evolved in Africa, evolving intelligence gradually enough so that the animals could evolve instinctive fears of humans, as being more dangerous than would be guessed by a naive mastodon, cave bear, giant sloth etc.

Thus the great herds of large mammals survived only in Africa?

How could one test or prove such a hypothesis? Or is there any way to know for sure what the answer is?


Questions that might be on the next exam and the final:

a) The arthropod phylum is one of how many phyla of multicellular animals?

b) When did the differences between these different phyla evolve?

c) Humans belong to which phylum? (hint: of which the vertebrates are the major subphylum)

d) Are sponges thought to be on the main line of evolution to the other, higher phyla of animals? What other phylum is?

e) Sponges are believed to have evolved from what other group of animals, based on possession of what strange cell type?

f) What did H.V. Wilson discover about sponge cells? (& when?)

g) What was Wilson's misinterpretation of this discovery?

h) What (over-simplified) explanation does our textbook and most people believe causes the phenomenon that Wilson discovered?

i) What related discoveries about sponges and sponge cells were later made by members of the UNC Biology Department?

j) What are some animals that are probably much more similar than sponges to the ancestral multicellular animals?

k) The most successful evolutionary branch of the animal kingdom has which phylum at the end of the branch?

l) Which two other phyla are believed to be fairly closely related to this phylum, and part of the same branch?

m) Besides vertebrates, what other kinds of animals are part of the second-most successful evolutionary branch of multicellular animals? (Please include two different groups)

n) What kind of skeleton do insects and other arthropods have?

o) In addition to insects, what are some of the other major sub-groups of the arthropods?

p) Why do insects and other arthropods have to molt in order to grow?

q) What are some similarities, and also some differences, between metamorphosis in flies as compared with metamorphosis in grasshoppers. Which is more like butterflies in this respect?

r) What 3 or 4 major evolutionary advances occurred as more advanced vertebrates evolved from the most primitive ones?

s) What are extraembryonic membranes? Hint: The developing embryos of which kinds of vertebrates form extraembryonic membranes? Another hint: What is a placenta?

t) Do all fish and reptiles lay eggs? (Hint: Most do, but not all.)

u) What species do we humans belong to, and about how long ago did this species evolve?

v) What living species of animal is evolutionarily closest to humans?

w) Our DNA base sequences differ from this other species by how many percent?

x) How does oxygen get to the tissues of ants, flies and grasshoppers?

*y) Have there ever been any other members of the same genus to which humans belong?

z) Did the first vertebrates have jaws? (hint: no) When did vertebrates first evolve jaws? Do any species of vertebrates, now alive, not have jaws? (Which ones?)

*!) Are clams evolutionarily more closely related to humans or to ants? What about starfish? Are they more closely related to ants or to humans? **What about barnacles?


The following questions WILL NOT BE ON THE EXAM because they are too difficult and open-ended, but are worth considering and discussing:

What are some alternative hypotheses to explain each of the following:
By what experiments or facts could they be proven?

What is the path of water flow in sponges?
Are there really incurrent canals?
Is water pushed along sheets of choanocytes?
Or is water pulled between choanocytes, from "mesohyl"?

Are sponges and evolutionary blind alley (as is now guessed)
Or are the other animal phyla descended from sponges, after all?

Is cell sorting caused only by selective adhesion proteins?
Or do other cell properties, like contractility, contribute?

    Three and a half:
Is the normal embryological rearrangements of cells caused by the same mechanisms as also cause dissociated cells to sort out?

Did the amnion and other extraembryonic membranes evolve originally in egg-laying species, for protecting the embryo etc.
OR did the amnion etc. first evolve in live-bearing species for the purpose of transferring nutrients from mother to embryo.

Did Homo sapiens interbreed with Neanderthal persons at all?

Did humans cause extinction of the large mammals of North America etc.

Why is Africa the only continent where the large mammals mostly did NOT become extinct?
Is the reason because humans evolved there?




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