Colibri:
That list seems to have some strange entries:
- Rodentia more modifed than Primates?
- Cetacea less modified than… anything?
Thanks!
And Starrman10, the clown line is from ‘Family Guy’, the episode where Peter and his wife run against each other for some kind of school position, and Peter’s old science teacher was the one who dissected the clown.
Colibri, could you say more about the logic used in creating the list you, uh, listed?
As an example, it seems to me that both Rodentia and Xenarthra are much more similar to the “low” end of your list than Primates. Admittedly I know little about these other than superficial appearance, but still…
I thought I read somewhere that the main reason for the brain of a dolphin being so big and complex was for signal processing … since they live in a three-dimensional world and have to process a lot of sonar data and the like.
(And of course, when I say “big,” I mean in relation to body size. Obviously elephants and whales and many other big animals have brains that are larger than humans, but rather than allowing said animals to churn out Broadway musicals, they’re mostly just used as control centers for the sizable bodies they reside in.)
[/humor]
They have…one of them is the current Secretary of Defense and another is the Attorney General.
[/humor]
That’s likely to be part of the equation.
They also have very complex social lives, much like primates. Whether that is a cause or an effect of a large, complex brain is open to debate.
John Mace and Algernon:
That’s a major problem with any taxonomic sequence - the evolutionary relationships are a complex three-dimensional tree that is impossible to represent in a linear sequence.
There are several things to be considered in formulating any taxonomic sequence, including both (1) branching pattern, (2) degree of modification. In general, lineages that branched off first are placed first; but within lineages that branch at the same level the less modified are placed first.
The problem is obvious in a group like the montremes - they branched off from the rest of the mammals very early, and although they retain some very “primitive” traits they are highly modified.
Perhaps the “Walker” sequence wasn’t the best one to use, but it was handy. “Walker” is a relatively traditional sequence; the higher-level classification of mammals is in a state of flux at the moment, so it’s hard to find a good sequence.
However, here’s a fairly recent tree (with the caveat that since it’s from 1995 it’s already a bit out of date).
Nowadays it’s generally thought that the edentates/pangolins are the most primitive eutherians (but of course, the anteaters/armadillos/sloths are all highly modified in some ways). After that, there’s little agreement on what order the other major groups branched off in.
Traditionally, Primates have been thought to be a relatively basal branch because of the evident relationship to the Insectivores (which are generally thought to be primitive) through the Tree-shrews. But this relationship is less clear than was once thought.
Relative to primates, rodents can be considered to be more “advanced” because of their extremely specialized and highly modified dentition - permanently growing incisors, loss of canines, etc.
Whales are indeed probably the most highly modified mammals. Their relative low position in the “Walker” sequence is probably due to the postulated close relationship to creodonts (now questioned), which were thought to be basal to the clade including carnivores and ungulates - so therefore whales are placed before these groups.
As I mentioned before, Primates are really relatively “generalized” mammals, with few major modifications of the skeleton compared to others. There are some adaptations for climbing, but primates generally have the primitive number of digits (five each on fore and hind limbs), have retained most of the original complement of bones present in ancestral mammals, have relatively unmodified teeth, etc. Few other groups besides the insectivores are so little modified from the basal eutherian body plan.
Um, I know a few who jump up and down, pull their hair out and scream blue murder when their theories are modified. I guess its those who let ‘ego’ take over?
Just to add a comment,
the common ancestor of both homo sapiens sapiens and the chimapnzee with whom we may share as much as 98% of our DNA is thought by paleoanthropologists to be “the picanthropus”. At least that was the culprit the last time I checked a couple years ago.
Now you know.
That’s what I get for running off my mouth.
From Stephen Jay Gould’s Wonderful Life:
“Chimps are not ancestors but modern cousins, equally distant in evolutionary terms from the unknown forebear of African great apes and humans. Pithecanthropus (Homo Erectus in modern terms) is a potential ancestor, and the only legitimate member of the sequence.” (page 29, hardcover edition)
I apologize yet again for sloppy speaking and misquoting.
I was taking a Gould quote out of context. He was not saying that Pithecanthropus was a common ancestor of chimpanzees and humans, homo erectus is much too recent for that.
However, what I should have said the very first time, If I had bothered to think before speaking, is that Ardipithecus is the closest specimen we yet have to a common ancestor.
And that is my last word, I promise.
re: relative “advancement” of rodents over primates
Consider the typical rodent dentition. The continously growing incisors are a derived feature which, in terms of divergence from the basal mammal pattern, are miles ahead of anything primates have come up with.
Absolutely true. I should have written that scientists don’t mind having someone else’s theory modified or overturned. In the interest of brevity, I gave a wrong impression that all were happy to see science advanced. I suppose ultimately all are glad to see that happen, but along the way some noses can get seriously out of joint.
In about 1830-40 or so, a Hungarian physician, Ignaz Semmelweiss, discovered and very cleverly proved, even by today’s standards, that the “disease” commonly called childbed fever was a result of contact infection from doctors’ hands. By insisting on conscientious washing in a solution of carbolic acid before and after the examination of each patient in his hospital he reduced the incidence of the disease to under 1% while in other hospitals it was as high as 40-50%.
Semmelweiss’ ideas were contrary to the conventional wisdom of the day and he was scorned and ridiculed by, among others, Robert Virchow who was a well known and able scientist. As a result Semmelweiss’ idea was ignored and not rediscovered until the time of Louis Pasteur and Joseph Lister.
The path of scientific discovery isn’t a smooth straight line from “good” to “better” but sooner or later “better” does come about.
Chalk one up for the rodents. But how about:
- larger brains
- binocular vision
- opposable thumbs
- small litters (usually only one)
- extended childhoods
- reduced sense of smell
These characteristics are not nearly as basic as changes to the skeleton or dentition. Primates, like any other group, have certain characteristics that define them. But they retain the fundamental body plan of primitive eutherians. The biggest skeletal changes in primates are probably the complete bony orbit around the eye. The evolution of opposable digits (found also in marsupials) is not as basic as the changes found (for example) in the ankle joint of artiodactyls. Another primate characteristic is a mobile shoulder joint (which involved the retention of the clavicle, which has been lost in many other mammals), but again, the modifications are relatively slight.
I might add that bats, despite their modifications for flight, are like primates in that many of their other skeletal and dental characteristics are relatively “primitive.” Primates similarly have a suite of characteristics connected to adaptation to a particular ecological role (arboreal living) but are otherwise primitive.