Wednesday, June 19, 2013

The Roots Of Our Animal Family Tree

Biology concepts – porifera, last common ancestor, placozoa, cladogram, lower metazoan, bilaterians

Bonobo apes (Pan paniscus) are very closely related to
chimpanzees. They have longer legs than common
chimpanzees (Pan troglodytes) and are also
distinguished by having pink lips. I think this makes
them look significantly more human-like. Also like
humans, the families seem to be run by the mothers.
Humans are descended from primates; we share 99% of our DNA with chimpanzees and Bonobos (pygmy chimps). But what do we find as we go farther back along the line of mammals, and then from animals in general?

Question of the Day:  What ancestor gave rise to all the animals and is it still around today?

This is a much tougher question than it would seem at first glance. When I was studying biology for the first time, I thought that since humans descended from apes, and we see apes, then apes must have diverged from some other animal type that we recognize – something like apes descended from rodents.

But evolution doesn’t have to work this way; not every group of animals has evolved directly from some other group of animals. At some point, mammals had a last common ancestor with some other group of animals, and before that, those ancestors had a last common ancestor with some older group, and so on until the last common ancestor was the organism that gave rise to the first animal.

So did me need a mammal to give rise to all mammals? It is much like - which came first, the chicken or the egg? We all know that dinosaurs were laying eggs millions of years before chickens, but try thinking of it like this – which came first, the chicken or the chicken egg?

In terms of evolution, there was some bird like animal that was almost what we would agree was a chicken genetically; let’s say it was missing just one mutation or rearrangement of genes that prevented it from being called a chicken. So this non-chicken lays an egg. The embryo inside just so happens to contain the very mutation or change that will let us call it a chicken. Is it a chicken – yes.  In a chicken egg – no. The chicken came first.

The chicken or the egg question is much more interesting
than most people realize. Consider what you call a chicken
egg – is it an egg from a chicken, or an egg that houses a
chicken? If you think it is an egg that develops around an
embryonic chicken, then the egg came first, as opposed to
the explanation in the text. I love discussion about what
words mean, they make us thinkers.
Our discussion of the non-chicken egg description makes it easier to imagine that there was some organism that, while not an animal, was mother to all animals. The question still remains as to how that animal might have looked or behaved – but that won’t keep us from looking at some possibilities.

It would be a nice feather in your cap if you were the person to discover evidence of the first animal. In 2012 there was one article that displayed fossils of track marks from possibly 585 million years ago – pushing back the previously accepted date of animals by 30 million years.

Yet there was another 2012 paper showing Namibian fossils that could be 760 million years old – pushing the start date of animals back more than 200 million years! The truth may be somewhere in between, or might be even earlier. However, fossils of the first animals, if they exist, would only give limited information. Can we look further?

The 760 million year date is in line with what some geneticists estimate for the first animal. By looking at genes that all animals have in common and the rates at which those genes change over time, scientists can backtrack to see when they might have emerged.

What if we look at today’s animals, and which may best represent the first animal. Are we talking about primitive animals? What does it mean to be a primitive animal? If an animal species was closely related to the last common ancestor of all animals, it would be easy to say that it was a primitive animal – it lived long ago when animals were new, and it had a lot in common with the first, most primitive animal.

But do not confuse a species or genus with an individual animal. We have animals today whose ancestors were very closely related to the first animal, but that doesn’t mean that these individuals are primitive – they could have undergone extensive evolution through the millennia. Quite a number of adaptations could have taken place that increase the complexity of the animals biochemistry and/or behaviors.

The last common ancestor is sometimes called the most recent common 
ancestor (MRCA). They both mean the same thing. This chart 
pinpoints the MRCA for all life on Earth. That does not mean that it 
gave rise to all life. There could have been several parallel lines that all died
off. Same for the animals – there could be whole animal
phylums we know nothing about.
On the other hand, we can look at organs and systems as a measure of complexity or primitiveness. All animals are classified as metazoans (meta = changing, zoa =  animal). Some are termed lower metazoans, because they do not have complex structures like spinal chords (chordates) or bilateral symmetry (bilaterians).

Organization makes animals more complex as well. Cells of different types can form tissues that have specific functions. Tissues can organize into organs and organs join together to form systems. Animals without these characteristics are termed “lower” or “simple” or “primitive.”

Likewise, animals that can’t perform behaviors that other animals can are supposedly more primitive. If one species can move while another can’t, then the sessile (non-moving) animal is more primitive. Nervous systems are supposedly a big feature of more complex animals.

These ideas can lead to great discussions relating cells to life. What does it mean for one culture to be more primitive than another. Does a lack of cell phones make you primitive? Amazonian cultures had been using certain medicines for thousands of years before we arrived and stole their pharmacology. Now who looks primitive?

All this being said, can we learn anything by looking at extant (living) species as representatives of what early animals might have looked like or how they might have behaved? Yes, I think we can. You can’t know where you are going if you don’t know where you’ve been.

Sponges might be a good place to start. Sponges are so primitive that most non-scientists don’t even think of them as animals. Most have no body symmetry, they appear to be sessile, and they have a very few cell types, none of which are organized into tissues or organs or systems.

Sponges have been around for about 760 million years, if we are to accept the Nambian fossils as well-dated and representative of the earliest sponges. This would put them in the front seat of the animal bus. But are they really that primitive?

This is the harp sponge (Chondrocladia lyra) that was discovered off 
the coast of Oregon, Washington state and Canada. It lives in the 
trenches below 3000 m and represents just one of the carnivorous
sponges. You want weirder, loo up a picture of the ping pong tree sponge!
Sponges generally have three different regions, the outer layer, the more acellular mesohyl, and the inner surface. Choanocytes line the inner surface and have a single flagellum that help the cell to harvest floating food in the filtered water. The outer layer is made up of pinacocytes that filter the water and digest food particles too large to be filtered. So far, interesting but not amazing.

But the mesohyl of this “primitive” animal has some cool stuff.  There are motile cells that secrete collagen protein. There are muscle cells that help the sponge contract and relax. There are “grey” cells that act as an immune system. And there are other motile cells that are totipotent stem cells and can become any cell type with in the sponge. Still sound primitive?

If you want a nice example of just how complex sponges can be, meet the harp sponge (Chondrocladia lyra). It is definitely a member of the phylum porifera (Latin for “bearing pores”), but it does have elegant symmetry. Described in late 2012, the harp sponge is also one of about 24 different carnivorous sponges.

The harp sponge uses sharp spikes on the vertical growths to harpoon and hold fish and crustaceans, which it then wraps them on a membrane and digests whole. This is in contrast to most sponges that filter microscopic food particles from the water by passing the water through its body from the outside and then up and out of its chimney (see video).

Trichoplax adharens may represent the most basal animal alive. Made
up of just a few thousands cells of only four different types, it has the
smallest genome of any known animal. The cutaway drawing on the
right shows that there are layers of cells, so it does have some
organization, just no tissues or nervous system.
The harp sponge is like other sponges in that it can reproduce asexually through fractured off pieces, by gemmules that are like clonal spores, or by budding. They can also reproduce sexually, but in the harp sponge the spermatophores are not simply released form the sponge body. They gather in the bulb portion at the top of the vertical shafts and are released all at once. The oocytes are found in the middle bulges. Sponges can reproduce four different ways while we only have one. We can, and will, spend more time on the exceptions that are sponges.

In recent years, less emphasis has been placed on sponges as a basal form of animal and more attention has been given to the placozoans (placo = flat, and zoa = animal). Only one species of placozoan is known (Trichoplax adharens) has been described, mostly because they have never been observed in their natural habitat (ocean, we think) and have only been seen on the walls of laboratory and zoological aquariums.

Placozoans have only four different cell types, no symmetry, two layers of cells, and no nervous system. Even by sponge standards, this is awfully primitive. The 2009 study of Schierwater et al. has given the best proof that T. adharens is the most basal of the lower metazoans, based on comparisons of thousands of genetic loci.  This agreed with several earlier studies, but Dr. Schierwater’s group went much further.

The cladogram on the left dates from 2009, showing that a more primitive
animal gave rise to both the lower metazonas and separately to the more
complex animals. The tree on the right is from 2013 symposium write up
in Integrative and Comparative Biology (doi:10.1093/icb/ict008), and
represents a consensus of the genetics data and opinions. They seem to
think that sponges diverged early than all the rest of the animals. Needless
to say, opinions vary.
Their cladogram evidence seems to indicate that sponges, cnidaria, ctentophora (comb jellies), and placozoans diverged as a single group and in parallel with bilaterian animals. Together, these data mean that as a group, the lower metazoans diverged from the more complex metazoans even before the emergence of sponges or placazoans (see cladogram). Complex animals did not evolve from sponges, jellies or placozoans at all – they came from some different ancestor.

So, this evidence suggests that there was something out there that was an ancestor of both the lower metazoans and the bilaterians, but was itself neither of them – an animal whose ancestor wasn’t an animal. Will we recognize it when we see it? It leads to another question. What will it have to have to be considered the first animal and not the last non-animal – just what makes an animal an animal?

Next week - how do stars determine the color of plants, and what colors might alien plants be?

Dohrmann, M., & Worheide, G. (2013). Novel Scenarios of Early Animal Evolution--Is It Time to Rewrite Textbooks? Integrative and Comparative Biology DOI: 10.1093/icb/ict008

Schierwater, B., Eitel, M., Jakob, W., Osigus, H., Hadrys, H., Dellaporta, S., Kolokotronis, S., & DeSalle, R. (2009). Concatenated Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis PLoS Biology, 7 (1) DOI: 10.1371/journal.pbio.1000020


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