Download Video Notes: Shape of Life III – Flatworms All animals need to obtain

Video Notes: Shape of Life III – Flatworms
All animals need to obtain food to survive. However, the active, directional pursuit and hunting
of food is not universal among animals, and had an identifiable evolutionary origin.
Palaeontologists are seeking clues in the fossil record about what these first hunters may have
been like. The earliest hunters had no bones, but did leave fossilized clues in form of fossilized
tracks in sediment. Detailed analysis of these tracks can reveal much about the size, movement,
and behavior of the animal who made them.
So, what kinds of animals were on the seafloor 540 million years ago (most of which have not
left intact, fossilized bodies)?
Recall that prior to this time, animals had already flourished for millions of years that were
largely fixed to the sea floor (sessile or sedentary). The first animals were like the sponges of
today. Then the flower-like cnidarians evolved, whose polyps could stretch, bend, and move, but
who could not pursue their food. Then, by 540 million years ago, a new kind of creature had
arisen that could move actively, with directionality and purpose. These first hunters probably
appeared around 565 million years ago, as revealed by their earliest tracks (a variety of other
active hunters had already evolved by 540 million years ago).
The fossilized tracks left by the first hunters around 565 million years ago are similar in size and
shape to a strand of human hair. What kind of animal was it?
Based on the tracks, this animal:
-was thin and cylindrical, with a rounded cross sectional shape
-had no appendages
-was capable of directional movement with intent
-had a head (cephalization) associated with its ability to move with directionality
Although we may never know exactly what this animal was (it was pretty clearly a worm of
some sort), there is no doubt about its legacy. Today, most animals are built on this same basic
design; namely, cephalization with associated bilateral symmetry. This design allows animals
to seek out what they need to survive, and is tremendously successful.
Today, there are living animals that belong to an ancient group that has changed very little over
hundreds of millions of years, and that are very similar to what scientists think the first hunter
was like. These are the flatworms (Phylum Platyheminthes). Today there are upwards of
20,000 species of flatworms. They are found in almost every environment, including inside
other animals.
The rise of hunters is one of evolution’s great success stories, but the story also involves quiet
destruction.
In Scotland, earthworms that aerate the soil and help drain pastures are responsible for the high
productivity of the soil. A flatworm recently introduced from New Zealand (perhaps brought
over in potted plants) is killing and eating many earthworms, and causing major problems. This
flatworm is spreading all over the United Kingdom.
Flatworms exhibit directional movement, using sense organs to detect and pursue their food.
Some predatory flatworms use chemical cues (e.g., the slime trails of earthworms), and also have
eyes that can detect the direction and intensity of light. The sensory organs are often in pairs,
giving the flatworm stereo senses that are more efficient for detecting the direction and distance
of various stimuli (these stereo senses are most highly developed in some of todays larger
predators, like leopards). These sense organs are closely connected to the central nervous system
– a brain – which is also located in the head, permitting faster processing of sensory information,
and faster responses. Muscles and/or cilia are used to produce directional movement of the
worm as it hunts. The flatworm’s sense organs and brain enable it to be able to make decisions
about when and where it will move in the hunt for food. Once food is located and obtained, a
protrusable pharynx leads into an extensively branched gut that reaches all portions of the
worm’s body (these animals have several organ systems, but a circulatory system for delivering
food and oxygen to the body tissues is not present).
It took millions of years to evolve, but some flatworms are now parasites.
Parasitic flatworms can grow up to 60 ft long in a human digestive tract. They have no eyes,
mouths, or guts. A special structure at the front end for attaching the worm to the wall of the
digestive tract, called a scolex, is composed of hooks and/or suckers. Each segment of these
worms makes sperm and eggs which are capable of self fertilization. One tapeworm can make
more eggs than the number of humans on earth. They need to produce this many fertilized eggs
to maximize the chances that a single one will go on to complete the life cycle, and give rise to
the next generation.
Interestingly, we inherited our own body’s machinery for sex from the first hunter as well. The
first hunter was also the first animal to evolve internal fertilization, a new way of delivering
sperm to egg. The ancestor of the flatworm combined the ability to move with a new way to
mate.
Free-living flatworms (most are benthic, but some can swim) in the tropics have interesting and
exotic methods attracting and fertilizing mates. They are all hermaphrodites. In an environment
where finding a mate is difficult, hermaphrodites have an advantage because they can mate with
any member of the same species they encounter. Some species engage in “penis fencing”, a
rather brutal practice where sex resembles warfare more than love. Each worm has two
dagger-like penises that they try to impale their partner with. The first one to successfully stab
their opponent and inject sperm is the winner; the loser has to play mother to its fertilized eggs,
using up valuable resources in the process. After the eggs are released, a new round of penis
fencing with another individual will follow.
Although 500 million years of evolution separates todays animals from the first hunter, we share
the same basic design. The basic genetic instructions that tell cells when, where, and how to
grow, first evolved in these worms.
There is a major question of interest for developmental biologists and geneticists - how are the
body’s patterns determined and controlled by genes?
Studies of fruit flies have revealed how many of these patterns are established, and have
demonstrated the presence of master genes that control pattern formation. These master control
genes, or hox genes, control the expression of other genes during early development. These in
turn tell cells when, where, and how to grow. Many of these insights came from the study of
mutations in fruit flies.
Other kinds of bilaterally symmetrical animals have now been studied to see if they, too, have
these same master control genes. This work has shown that they do, and that similar genes in
very different organisms make similar structures. These pattern forming genes are inherited
from a common ancestor. The common ancestor that had these genes was, most probably, like
the modern flatworms.