Why did some freshwater fishes decide to move on to the shore but marine ones didn't ?
If you think of it, the answer is simple: Ocean is such a big and nice place that its inhabitants never needed to consider any drastic move. If a part of the sea becomes not suitable for their subsistence, the fishes could always swim to somewhere else.
But, the environment of freshwater is different. Many lakes dry up intermittently - leaving just an area of swamp; rivers often run a slightly different path every few years and the level depends very much on the rain fall (or snow fall). Vertebrates swimming in this kind of environment are forced to get up on (alas, develop) their own feet.
PS. Of course many fishes could not adapt to the change and died of dessication. It was fortunate that our pantheistic god is not some modern hospital administrators - otherwise these terrestrial beginners would be confined to the water forever because their apparatus (alas, fins) would always be considered not suitable for walking.
Saturday, February 14, 2009
Friday, February 13, 2009
Water
My discussion yesterday may seem entirely reasonable and logical, but there is one major loophole in the assumption - and the actual fact as well.
The very point is: terrestrial vertebrates did not come from marine animals, but freshwater ones.
Of course freshwater fishes originated from the sea. Nonetheless, by settling in an environment free of sodium chloride, the animal is adapted to an entirely opposite logic to handle minerals - to conserve ions, and get away with the water.
(Honoured to be a nephrologist, the latter adaptation results in the development of glomerulus - the very structure that filters a huge volume of blood and excrete it as some dilute solution. Seawater fishes, on the other hand, have no glomerulus; metabolic waste is actively secreted by kidney tubules - an ability that most terrestrial animals still preserve.)
The real problem, therefore, becomes: why should freshwater fishes - rather than marine ones, which obviously out-numbered the former - get to the shore and start walking ?
The very point is: terrestrial vertebrates did not come from marine animals, but freshwater ones.
Of course freshwater fishes originated from the sea. Nonetheless, by settling in an environment free of sodium chloride, the animal is adapted to an entirely opposite logic to handle minerals - to conserve ions, and get away with the water.
(Honoured to be a nephrologist, the latter adaptation results in the development of glomerulus - the very structure that filters a huge volume of blood and excrete it as some dilute solution. Seawater fishes, on the other hand, have no glomerulus; metabolic waste is actively secreted by kidney tubules - an ability that most terrestrial animals still preserve.)
The real problem, therefore, becomes: why should freshwater fishes - rather than marine ones, which obviously out-numbered the former - get to the shore and start walking ?
Thursday, February 12, 2009
Sea
One niggling problem on evolution that troubled me for some years is: Why couldn't we - most of the terrestrial vertebrates, I mean - drink seawater ?
On the face of it, the ability is obviously advantageous: the supply of freshwater is very much limited as compared to some sodium chloride solution.
But we could not.
The simple answer is, although there is a surplus of sea water on earth, most of the in-land source, i.e. river and lake, is freshwater. To explore the land, terrestrial animals must be able to drink pure water.
But that's beyond the point ! If animals on land originated from marine ones, they should be able to drink seawater to start with. They may gradually learn how to tolerate lakes and rivers when the species explore the land, but what we are seeing is their selective loss of an ability that they used to have - and a very valuable one in terms of competition.
On the face of it, the ability is obviously advantageous: the supply of freshwater is very much limited as compared to some sodium chloride solution.
But we could not.
The simple answer is, although there is a surplus of sea water on earth, most of the in-land source, i.e. river and lake, is freshwater. To explore the land, terrestrial animals must be able to drink pure water.
But that's beyond the point ! If animals on land originated from marine ones, they should be able to drink seawater to start with. They may gradually learn how to tolerate lakes and rivers when the species explore the land, but what we are seeing is their selective loss of an ability that they used to have - and a very valuable one in terms of competition.
Wednesday, February 11, 2009
Prediction
One remarkable story about Charles Darwin I learn elsewhere recently is his prediction of the existence of some gigantic moth:
"In madagascar there must be moths with the proboscis capable of extension to a length of between ten and eleven inches ... The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop."
Yes, that's a wonderful theory and logical prediction. Nonetheless, when the author of On the Origin of Species died in 1882 (still believing in the existence of this extraordinary moth), no trace of it had never been found ...
... not until forty years after its original prediction.
It was officially named Xanthophan morgani praedicta - in honor of the prediction by the man we shared the legacy today.
PS. The masterpiece of Charles Darwin took (10 months to write but) 26 years of data collection; his prediction took 40 years to be proved. Both are impossible happenings nowadays - even if he were awarded of some research grant, what could he put up in the progress report every year ?
"In madagascar there must be moths with the proboscis capable of extension to a length of between ten and eleven inches ... The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop."
Yes, that's a wonderful theory and logical prediction. Nonetheless, when the author of On the Origin of Species died in 1882 (still believing in the existence of this extraordinary moth), no trace of it had never been found ...
... not until forty years after its original prediction.
It was officially named Xanthophan morgani praedicta - in honor of the prediction by the man we shared the legacy today.
PS. The masterpiece of Charles Darwin took (10 months to write but) 26 years of data collection; his prediction took 40 years to be proved. Both are impossible happenings nowadays - even if he were awarded of some research grant, what could he put up in the progress report every year ?
Tuesday, February 10, 2009
Darwin
Read the yearly supplement of The Lancet during the weekend. The subtitle of this year is Darwin's Gift - it is a collection of papers celebrating the bicentennial birth on the giant in evolution.
(The supplement is, somewhat surprisingly, the most enjoyable one in the past five or six years.)
It is often stated that the author of On the Origin of Species was the pioneer of the theory of evolution. That's not entirely correct - the hypothesis of evolution had been around for half a century before his time. His real breakthrough was the proposal of natural selection as the cause of evolution. The theory always reminds me a saying of Arthur Schopenhauer:
All truth passes through three stages:
(The supplement is, somewhat surprisingly, the most enjoyable one in the past five or six years.)
It is often stated that the author of On the Origin of Species was the pioneer of the theory of evolution. That's not entirely correct - the hypothesis of evolution had been around for half a century before his time. His real breakthrough was the proposal of natural selection as the cause of evolution. The theory always reminds me a saying of Arthur Schopenhauer:
All truth passes through three stages:
- First, it is ridiculed.
- Second, it is violently opposed.
- Third, it is accepted as being self-evident.
Monday, February 9, 2009
Quiz
It is the Chinese Valentine's day today. I shall put up a quiz for you to ponder:
The shortest way to walk from place A to B is to go south for two miles, and then to the north for two miles. How far do you have to walk from place A to B by going east ?
I may well provide a Chinese translation: 從A地到B地最短的方式是先南向行兩英里，然後向北行兩英里。問:從A地到B地，往東行有多遠?
PS. Don't be paranoid. This is a question on mathematics, not some leg-pulling IQ test.
The shortest way to walk from place A to B is to go south for two miles, and then to the north for two miles. How far do you have to walk from place A to B by going east ?
I may well provide a Chinese translation: 從A地到B地最短的方式是先南向行兩英里，然後向北行兩英里。問:從A地到B地，往東行有多遠?
PS. Don't be paranoid. This is a question on mathematics, not some leg-pulling IQ test.
Sunday, February 8, 2009
Poison
Having a lovely book The Science of Sherlock Holmes, a gift from KM, as my bedtime reading.
In essence, it is a book on the history of forensic medicine, and covers various aspects of the discipline - poison, firearm, footstep tracing, blood stain identification, and what not - with frequent reference to the sayings and doings of our great detective. To an admirer of the Victorian London, it is simply an eye-opening experience.
A small point that I noted while thumbing through this enjoyable volume was the easy availability of poison a hundred years ago. In fact, senior medical students were allowed to prescribe and it seemed a custom to ask these later-to-be-physicians to treat simple conditions.
Of course, the down side was it also provided an excellent means to do away with any difficult wife or mother-in-law.
But, to our friends outside the solar system: Yes, students did make treatment orders, not to say house officers.
As Publius Vergilius Maro said: Possunt, quia posse videntur.
In essence, it is a book on the history of forensic medicine, and covers various aspects of the discipline - poison, firearm, footstep tracing, blood stain identification, and what not - with frequent reference to the sayings and doings of our great detective. To an admirer of the Victorian London, it is simply an eye-opening experience.
A small point that I noted while thumbing through this enjoyable volume was the easy availability of poison a hundred years ago. In fact, senior medical students were allowed to prescribe and it seemed a custom to ask these later-to-be-physicians to treat simple conditions.
Of course, the down side was it also provided an excellent means to do away with any difficult wife or mother-in-law.
But, to our friends outside the solar system: Yes, students did make treatment orders, not to say house officers.
As Publius Vergilius Maro said: Possunt, quia posse videntur.
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