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Overview

Unfortunately, we tend to think of parasites as the lowest form of life. They’re creatures that don’t contribute anything and take advantage of others.

Our view of parasites has been limited and biased. It’s time we realize that they’re critical to the ecosystem, and if anything, we should be thankful for them. This key points will give you an intimate look into this fascinating world through stages of life and metamorphoses to strategies used by parasites in order to survive on our planet. You’ll find out how some turn their hosts into zombies; why many castrate their hosts; and why you should thank a parasite for its role in the evolution of sexual intercourse.

Big Idea #1: Parasites often pass through many life cycles before they reach their final form.

Almost every animal on the planet will one day have a parasite. Despite their ubiquity, scientists didn’t understand them for thousands of years.

In the past, people thought that parasites spontaneously generated inside bodies. This belief was based on a lack of information about how they spread and reproduced. Now we know that parasites are highly complicated organisms with life cycles unlike anything humans have seen before.

In the 1830s, scientists began studying flukes to find out how they reproduced. They observed that flukes laid eggs, but nobody had seen a baby fluke in its host.

Steenstrup’s experiments showed that the flukes found in sheep’s livers were actually a final stage of an animal’s life cycle. He observed that eggs laid by adult flukes within their hosts escaped and hatched into larvae, which appeared to be covered with fine hairs and swam around until they penetrated snails.

Once inside the snail, these parasites transformed into a shapeless bag that was swollen with embryos of more flukes.

The King’s yellow worms turn into missile-tailed cercariae, which then transform themselves into mature flukes. Armed with that knowledge, scientists can finally discard the notion that parasites generate spontaneously.

Big Idea #2: Despite what some scientists believe, parasites are highly complex creatures.

Parasites are often considered disgusting. In fact, before scientists started studying them, they were looked down upon. This was epitomized in 1879 by the zoologist Ray Lankester when he said that parasites shunned evolution and developed backwards. He saw life as a Christmas tree-shaped hierarchy with humans at the top and other species below us on branches.

Lankester also added branches that drooped to represent species that degenerated over time. He believed that degeneration occurred as species became less complex than they had been in the past.

One of his favorite examples is Sacculina carcini, a parasite that appears to be a barnacle when it’s outside the host. Once inside the host, however, it loses its legs and tails.

Lankester’s views on parasites led to a lack of research into their complexity, which was later rectified by Sukhdeo.

Sukhdeo dedicated his life to studying parasites, which are organisms that live in or on other organisms. One of the main questions he investigated was how they navigate within their hosts’ bodies and reach their final destinations, such as an organ where they make a home and lay eggs.

The author came to the conclusion that parasites have inherited behavior, which is more than animals.

Despite his groundbreaking research on parasites, Sukhdeo had to endure disrespect from researchers who were more interested in animal behavior. They focused on vertebrates because they believed that parasites seemed too primitive to have any kind of behavior at all.