Sunday, May 13, 2012
(Picture from here.)
Here's a very good article on the evolution of the animal developmental process. It's not so good at explaining plant embryogenesis (wikipedia article here.) If you followed the articles conclusions, plants would be a form of degenerate animals since they also have embryogenesis. I'm not going to go into it in this post but I suggest reading it.
My own suspicion is that the process of multicellular organization that led to the evolution of development probably predated either plants or animals. The organizational principles might have been in play (see Volvox for example) but the actual process of developing embryogenesis likely have evolved separately between the plants and animals. When I read about it I see little similarity other than the fact that both higher plants and animals have a definable pattern of developing adult organisms from dissimilar embryonic precursors.
The evolution of fungal life cycles and that of slime molds likely also have evolved separately. Fungi look like they have evolved from animals or at least an animal precursor. (See here.) There is some genetic evidence that the gilled mushrooms and puffball fruiting bodies may have evolved fairly recently and may also have arisen separately as many as four times. (See here.) What's less clear is how fruiting mechanisms of fungi evolved at all.
Slime molds (Myxomycetes) are interesting in that they have a single celled form and a multicelled form. When they mate they form a plasmodium, a large structure consisting of nuclei without any intervening cell membranes. The plasmodium then extends itself (and sometimes appears to actually move) by cytoplasmic stream and consumes the food source. When the food source wanes, it forms a fruiting body that looks mushroom like and develops spores.
These sorts of strategies are important in Science Fiction. SF aliens even in their most extreme form are usually animal in nature. There are a few exceptions: the living planet in Stanislaw Lem's Solaris. And the microbial gods of of Ken MacLeod's Engines of Light trilogy. SF Plant intelligences are essentially animals in plant suits. (See here.) Which is only marginally better than humans in alien suits.
It doesn't have to be that way.
I'm a big fan of using the strange and wonderful biological world of earth as inspiration for alien worlds. And, certainly, the animal kingdom is filled with the truly strange and completely non-human. But I'm also among the first to admit that this is animal chauvinism.
We have no idea of the shape of life on other worlds. I think we can suggest that if there are the ingredients and opportunity for a carbon based origin of life that life will in fact arise. And we can further speculate with evidence that the result will go as far as a similar metabolic path as earth's: using light to fix CO2 into carbon polymers and thereby releasing oxygen and metabolizing the resulting organic compounds in the presence of the resulting oxygen.Note the caveats: it says nothing about non-carbon life. We can't even say that life has to take a cellular path-- there are some very interesting experiments involving organic compounds unconstrained by chemical cells but constrained by physical components such as the cavities in lava rock. Or that individual cells are the preferred path. The slime mold plasmodium mentioned above is fairly large-- sometimes meters across-- and has no cell membraines. It is, in effect, a single cell.
We have no idea what things are like out there.We know only a little more about what things are right here.
The stories we want to tell are about human beings. Aliens are one wrench in the tool chest writers can use to tinker with the human experience and see how it works.This is, in my opinion, one of the distinctions between SF and fantasy. SF has at least the toe of one foot in the world we live in, the world of thermodynamics and the Cretaceous. Fantasy does not need that toe. In a fantasy world we can have creatures that pursue evil for no reason except they're evil. In the SF world, evil villains at least have to pursue evil because it makes them look cool. This is a technical evaluation not a judgement of quality. It's just a distinction. A work of fantasy does not have to conform to physics though it may choose to. A work of SF is informed by physics even if the physics are ignored.
I've been talking about biology for a bit now. My intention is not just to say Look at that! Isn't it cool? though that is certainly true. Instead, I'm saying that the biological world can serve as a tremendous well of metaphorical construction. Along with history, anthropology and archaeology, biology can help determine motivations, develop character and make stories.
Even with slime molds.
Saturday, May 5, 2012
(Picture from here.)
One of the things that changes over time is our perspective on our ancestors. It wasn't so long ago that we thought of Neanderthals as stupid, shambling brutes. Now we know they may have had brains that rivaled ours, speech and red hair. Our perspective of our ancient ancestors is also undergoing a revision.
Lucy is one of our earliest relatives. She's a representative of Australopithecus afarensis from about 3.2 million yers ago. Two years ago a new species of Australopithecus, A. sediba, was discovered also in Africa. A. sediba has a brain that is more organized like a human's than others.(See here.) She also has a tool maker's hand-- a hand much more capable of using tools than other members of her genus. However, she also has a foot that is more primitive than Lucy. (See here.) A. sediba is dated to about 2 million years ago.
Human beings belong to genus Homo. Home first appears with Homo habilis 2.33 to 1.8 million years ago.Which make one wonder if there was an overlap between the origin of Homo and the existence of A. sediba. It certainly makes the origin of genus Homo more interesting. Possibly, we'll discover another member of Australopithecus that will join the bipdal gait of Lucy with genus Homo.
The rhino in the room for this discussion is what traits are derived traits that humans inherited from non-human ancestors and what humans evolved on their own.Traits that we identify as hallmarks of our humanity include bipedal gate and the human brain. A. afarensis was fully bipedal but didn't have a human brain. A. sediba had, organizationally at least, something that might have evolved into a human brain but didn't have a human foot to back it up.
Adding to that is a recent find of another Australopithecus, a contemporary of Lucy, that had more of a tree dwelling foot than Lucy did. (See here.)
Questions about. Did Lucy or another Lucy-like relative serve as the ancestor of Homo habilis and the brain organization derive separately from A. sediba? Or did A. sediba, or something like her, serve as an ancestor to Homo habilis and the foot derive separately? Or, is there a third Australopithecus species waiting to be found?
What we've been calling late derived traits are being pushed back all the time. Baboons had been shown to be able to distinguish gibberish from actual words-- an astonishing feat for a species that doesn't talk. (See here also.) Clearly, they are not determining this by the meaning of the words. Instead, they must be using some sort of statistical mechanism to detect patterns. Humans probably do the same thing. However, the lineage from which humans arose split from the baboon line thirty million years ago. Baboons
My point is the the dividing line between what an ancient genus Homo and an ancient genus Australopithecus may not have been all that great.
A new study that came out in the last few weeks looks at the number of large predators over time. These days there are only a few predators over 70 kg. But as recently as three million years ago there were many more. About two million years ago the number of large predators crashed. The study ruled out climate change or other more typical "natural" disasters. But it couldn't rule out that humans, or equivalents, might have done it.
The granularity of the time domain is broad-- 500k years. Which is problematic since if the actual date moves one way or the other genus Homo might not be responsible. If it's too early, Homo wasn't around to do the job. If it's too late then Homo was around for quite a while without making any impact.
But as I read the article, I kept remembering A. sediba. Baboons. Chimps. And all the other animals we've arrogantly claimed couldn't use tools, communicate or think.
If humans didn't do the deed there might still be other candidates.