✔ 最佳答案
generally opinions on this are speculation, since most of this kind of evolution occurred millions of years ago, and organs dont preserve like bones, i doubt anyone truly knows the method but lets take a shot with the digestive system
we know the vomiting digestive enzymes onto food before ingesting it was probably the first method for 'eating' as many flies still do it today, the evolutionary steps may have gone like this:
1 - organisms simply ingest undigested organic matter, such as sugars which don't need as much processing, the ingestion would occur through endocytosis, these are unicellular or very basic multicellular organisms,
2 - digestive enzymes to break down proteins fats etc begin to arise, this confers an advantage to the orgaism as it doesnt have to find readily available amino acids that it can just incorporate it can break down proteins and then absorb them, like a venus fly trap
3 - the animal gets a form of gut where it stores food while the digestive enzymes break it down, DEs are not produced in the gut in this case tehy are produced seperately and secreted onto the food, this confers a higher uptake efficiency as the food stays with them for longer, and the organism can carry on hunting while it digests
4 - DEs start to be secreted in the gut, probably vomits aswell, this increases digestion efficiency and uses less energy and waste as vomiting is reduced, reducing the loss of bodily fluids
5- from here on parts of the gut will start to compartmentalise and specialise, each focussing on digesting a certain aspect, being much more efficient at it
the cardiovascular system may be easier to explain as evolution of it is halted in many organisms as they have vastly differing demands for oxygen:
Bacteria - absorb oxygen from the environment, only organelle involved is the cell membrane -dont need very much oxygen
annelids (worms) - develop a heart and form of circulation, still absorb oxygen through their skin but now a blood gradient is kept up by constantly pumping oxygenated blood around, not very big so they dont need to increase surface area/ put effort into oxygen aquisition
roaches and other flies/ soft fish : have trachae / gills to increase the amount of oxygen that is supplied to their blood, higher surface area higher rate of exchange, since flies are small, actively breathing in air would be a waste of energy, its more efficient to have extensive ventilation and let the wind do the work
amphibians - they breathe ie have lungs, although this costs alot of energy compared to ventilation which requires none, it vastly increases their oxygen supply, and they cant rely on the natural underwater current to keep the oxygen flowing while on land, which fish can underwater (sea currents) the blood circulation system is also improved
reptiles - amphibious reptiles need to be able to hold their breath as they cant absorb oxygen through their skin like amphibians, requiring bigger lung muscles, and the evolution of the diapraghm.
mammals and birds - they run around have sex, fight and do all sorts of stuff to a much greater extent than any other kingdom and need the most oxygen for doing all this so they have the most powerful circulatory system with the heart being split into four chambers, and developing both arteries and veins which amphibians lack (their blood just goes round the body heart-lungs-body-heart instead of heart-lungs-heart-body-heart) and they have the strongest ribs and intercostal muscles
I hope that helps explain it all to you, i could talk about bacterial evolution aswell but thats a whole other story, just remember that each little baby step is beneficial in its own way, theres no huge jumps inbetween and the prcess of specialisation takes millenia
