why most of the creatures are smaller than their ancestor?

Question

i read a science section on a newspaper today. it shows some people found some fossils, one is a giant dragonfly is as big as a bird with the wingspan of more than 2 ft wide. Holy Bug! also, a saber-tooth cat skull with 2 huge long teeth. that makes me think why all the insects/animals were much larger than today.
for example, like Mammoth, although elephants are the largest land animal, compare to mammoth, its like the comparison of an elephant and cow. Also, others like giant crocodile, dinosaur(now became little lizard. komodo dragon, i believe dinosaur is their ancestor), cats had longer tooth, even insect were much larger. what was the main cause? Global temperture? Foods? or something else?

Answer 1

Well, many Creationists believe Noah's Flood would have drastically changed the climate of the earth (cooler temperatures, more radiation, etc.), and that there was a short ice age after the flood. Some Creationists think the dinosaurs that survived the flood where not able to cope so well in the new environment. They also believe that is the reason why man’s life-span shortened so much after the flood, and why animals are so much smaller.

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Answer 2

Perhaps part of the reason is a matter that the totality of a creature is in its volume. That will scale as a cube. For mammals at least, a big part of the immume system is the skin which, as the surface area, scales as a square. The sensitivity of the immune system to an increase in scale may have favored the smaller in their adaptation. They also consume less to sustain themselves. OTOH, there is a limit, especially dependent on the creature's diet, on the lowerbound for success as a hunter and for self defense as part of that daptation. So, in the end, with whatever variables in play, it comes down to an efficiency in the adaptation. That's really the crux of evolution anyway.

Answer 3

Varied speculation - probably because there are layers to the reasons why species don't survive, and no magic formula for all of them. That being said, I think the "largeness factor" can impact the demise of a species because there is an over-specialization of the species, a compounded mutation making them too big for their own good. The more specialized a species is, the more dependent the species is on the other animals, organisms, climate, etc. of its specific environment and less able it is to adapt to a changing environment. For example: the now extinct dire wolf was 5 feet long and 150lbs. (appx. 50% larger than the gray wolf), had very sharp teeth and strong jaws, but they had short legs - Good for power and balance on ice, but thought to be too slow to survive as predators after the last ice age. Food supply is definitely an issue as the world evolves along with shrinking/shifting of hunting and/or breeding grounds... Perhaps the "super crocodiles" met their demise because the introduction of clever rats/rodents/low mammals decided the eggs of the larger crocs were delicious, and provided more sustenance than the eggs of the smaller crocs - Or perhaps the metabolisms of the mega crocs weren't efficient enough based on the shrinking of the food supply and overall diminishing landscape (contemporary crocodiles are excellent killing machines, but do not have to eat as much as you would think). Of course, larger animals are more prone to influence from environmental changes like disease or pollution (look at the mercury levels in the larger fish in the ocean). I think the trend for one species to dominate can be challenging to the diverse laws of nature, and balance should be considered or it will be inevitably self-corrected.... Corny as it sounds, I think that teamwork plays a major role to the survival of a species. Let's call it social interdependence. The rats that we talked about earlier - pretty efficient species, don't you think? The smaller crocodiles - the ones still around today - were also probably better suited to protect their eggs, as they have more in numbers to ward off the predators of their eggs. Elephants are large (not like mammoths, who were prone to warming planet and significantly hunted), but they are highly social and interdependent. Same could be said for whales.... I also think that the smallness of our ancestors played into their healthy fear of larger predators. (Have you ever been in a building that was made even 500 years ago - It's small!!!) We too are getting larger with better diets (contrary to popular belief), better health, mastery or our environments, etc. Lesson to be learned: social interdependence. Teamwork. It probably wouldn’t hurt to stay small and scrappy. ;-)

Answer 4

Predation is also a factor. Of animals are heavily preyed upon then a natural response is to begin breeding from an earlier age. If this is to happen then they must give birth to smaller young typically resulting in a smaller adult.

Answer 5

Not enough room to grow. Most open lands have become populated with people so the animals have to adapt and getting smaller is a necessary evolution.

Answer 6

I think animals just adapted to their enviroment...that is why they are dramatically smaller. The earth has changed alot since that time...i guess with everything..you evolve to survive.

Answer 7

What about birds? they came from dinosaurs.