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Asked by ILuvScience to Mark, Koi, Claire on 14 Jun 2016.
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Mark Booth answered on 14 Jun 2016:
Hi Iluvscience
Parasites have a number of different body shapes and life cycles that are the result of evolutionary pressures. It is probably true that most parasites started off (millions of years ago) as free-living organisms that mutated in such a way that they could exploit the resources that a host provides (this could be as ‘simple’ as worm attaching to a host for protection or transport that evolves a way of getting into the host).
There are several classes of parasite that have evolved body shapes and sizes that correspond to their resting place within the host. So we find roundworms like ascaris in the intestines, and small, flagellated parasites like the trypanosomes swimming in blood
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Arporn Wangwiwatsin answered on 15 Jun 2016:
Hi ILuvScience,
Yes, I’d go with Mark’s answer that it’s come as a result of evolutionary pressure to take advantaged of the host …. now after evolving into certain forms over a long long long time, during which time the information on how to form embedded in their genome (which is the blueprint of how an organism would work), then the next question is how they control their form over their life span. … Some worms, when they first came out from eggs, look quite different from when they are adults, sometimes parasitic worms also have stage that look nothing like worms!; and parasites such as Plasmodium (one that cause malaria) go through a cycle of different forms. It is still a question on what control the form changing within one type of parasite at different time… There are still a lot to find out.
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Claire Bourke answered on 16 Jun 2016:
Great answers from Mark & Linda there.
One of the amazing things about evolution is just how specialised the results can be. For example, in the case of schistosome worms, which are parasites that live in the blood stream, the female worm has evolved to be very very slender so that she can precisely position her eggs into narrow capillary blood vessels so that they have a better chance of escaping the human body. To allow the female worm to be so slender she cannot have the bulky tissue she needs to be able to move around easily, so the male worm has developed a groove in its body that the female can fit inside so that it can support her inside the blood stream. Imagine how long it would take to develop such a complex combination of shapes and sizes even in this one parasite and then think of all of the different shapes and sizes and life cycles of all of the different parasites; truly mind-blowing!
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Franco commented on :
Mark and Koi explained this very well. I just wanted to add that the separate life stages and the life cycles of the parasites can be so different, that in some cases it took researchers a long time to realise that what they and others were looking at were different life stages of the same parasite!
So life cycles are very complex and involve several very different hosts, so it’s not very obvious and requires thorough investigation.