For Tom, Ellen and Robert, who make me me ! First published 2013 by A & C Black, an imprint of Bloomsbury Publishing Plc 50 Bedford Square, London, WC1B 3DP www.bloomsbury.com Copyright © 2013 A & C Black Text copyright © 2013 Gill Arbuthnott Illustration copyright © 2013 Marc Mones Additional picture acknowledgements: Additional images all Shutterstock, aside from the following: p6 top right and p58 left © Wikimedia, p6 bottom left and p58 top right © Wikimedia, p11 bottom right and p58 left © Wikimedia, p16 top left and p58 right © BE031947/Corbis, p16 top middle and p59 left © King’s College London, p16 top right and p58 right © BE031947/Corbis, p17 top right and p58 bottom © Jenifer Glynn, p19 top left © Science Source, p19 top left © Science Source, p29 bottom right © Wikimedia. The rights of Gill Arubthnott and Marc Mones to be identified as the author and illustrator of this work respectively have been asserted by them in accordance with the Copyrights, Designs and Patents Act 1988. ISBN 978-1-4081-9406-5 A CIP catalogue for this book is available from the British Library. All rights reserved. No part of this publication may be reproduced in any form or by any means – graphic, electronic or mechanical, including photocopying, recording, taping or information storage and retrieval systems – without the prior permission in writing of the publishers. All papers used by Bloomsbury Publishing are natural, recyclable products made from wood grown in well-managed forests. The manufacturing processes conform to the environmental regulations of the country of origin Printed in China by C & C Offset Printing Co., Ltd. 10 9 8 7 6 5 4 3 2 1
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Charles Darwin and Evolution
Charles Darwin developed the Theory of Evolution by Natural Selection .
Charles Darwin (1809-1882) was interested in all sorts of living things. He was fond of worms and pigeons, but he really loved beetles. When he was young he was out looking for beetles one day and spotted one he didn’t have, so he picked it up. Then he found another new one, so he picked that up in the other hand. Then he found yet another new one, but he’d run out of hands. So what did he do? He put one in his mouth, of course, so he could pick up the third one.
Here’s how it works:
Some sorts of animals and plants only have a few babies. Others have lots. Some have millions. So why hasn’t the world filled up with rabbits or rats?
Darwin spent his whole life studying all sorts of living things, and made many discoveries, but the one he is remembered for is called the Theory of Evolution by Natural Selection. It explains how all the living things in the world today developed over a huge length of time from a few, much simpler living things. 6
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The answer is that almost all the babies die before they can breed. For most sorts of living thing, this means the total population stays about the same all the time.
N a tu ra l s e le ctio n
Th is ha pp en ed ov er an d ov er ag ai n fo r mi lli on s of ye ar s – no t ju st w it h ra ts , bu t w it h al l liv in g th in gs . Th e wo rl d st ar te d of f w it h ju st a fe w si mp le an im al s an d pl an ts , bu t mo re an d mo re ap pe ar ed as ti me we nt on . No w th er e ar e mi lli on s of di ff er en t sp ec ie s.
The babies – let’s say baby rats – are all different from each other. Some can run faster than others, have a better sense of smell, or have sharper teeth. They all compete with each other for things like food, shelter and mates. The ‘best’ rats find the most food and the safest shelters, so they live long enough to breed. The ones that are slower, or less good at sniffing out food, die.
One question Darwin couldn’t answer, however, was why the rats were different from each other in the first place?
The rats that breed pass on what made them the ‘best’ ones (being good at running or biting or smelling) to their babies. This goes on and on over many years, and the rat population slowly changes into… super rats! They’re so different from the original rats that they have become a new species.
We know now that it’s because of their DNA.
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What does DNA do? DNA is a set of instructions for making a living thing. Each instruction is called a gene. Each gene controls one thing, like eye colour or hair colour or nose shape. The order of the bases on the DNA makes up each gene. Many genes together make up each chromosome. It seems incredible that something as simple as DNA can make you, or a tree, or a frog. DNA was discovered in 1869, but it was ages before anyone realised how important it was. A four-letter alphabet seemed much too simple to control everything that happened inside a cell. The next surprise about DNA is that only about ten per cent of it actually makes up genes. The ‘junk DNA’, as we call the non-gene stuff, is still very important though. Scientists now think that it helps to switch genes on and off at the right times. We also use junk DNA for DNA fingerprinting.
There’s more about DNA later in the book.
A (Adenine) C (Cytosine) 9
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So What is this DNA Stuff Any way? What is DNA?
Your cells are too small to see without a microscope. Five hundred of them side by side would only measure one centimetre. But you have two metres of DNA in each cell! Altogether you have thousands of kilometres of DNA in your body.
Your body is made up of billions of cells. Every cell contains long threadlike things called chromosomes , made of a substance called DNA . DNA is very, very long and thin. It’s shaped like a twisted ladder (sometimes this is called a Double Helix ).
It must be pretty important then . . . You need to understand a bit about DNA before the rest of this book will make sense. Here goes . . .
The rungs (steps) of the ladder are made of pairs of molecules called bases . There are four different bases, called A , C , G and T. They fit together in pairs. A fits with T, and C fits with G. The order of the bases down one side of the ladder forms a code – a bit like an alphabet that only has four letters in it.
T (Thymine)
G (Guanine)
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The Monk and the Pea Plants At about the same time as Darwin was working out his ideas about evolution, Gregor Mendel was counting peas‌ Mendel (1822-1884) was a monk in a monastery in what is now the Czech Republic. He wanted to be a Biology teacher as well as a monk, but he failed his exams twice, so he had to change his plans and teach maths and physics instead. (Is that really easier?) As well as teaching and doing all the things that monks were meant to do, he was in charge
of the monastery gardens. He spent a lot of time doing experiments on pea plants. He was interested in how things like the colour of the flowers or the length of the stems passed from one plant to its offspring.
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did: l e d n e M t a H e r e’s w h from one of t o o k p o ll e n
t He la n t s a n d p u f lo w e r s 4 . p e ll h a t t m w o e r n f o ll e n the se of anot he r it s t r 1 . He took p u e p w d lo f n a e t h p la n it in t o t a e p o f a t a ll p e a t r o h s a t a ll p la n t . we r s of w e lo n f e h t o t in ds of t hat e e s e h t d e p la n t . 5 . H e c o ll e c t e m t o s e e if h e t h f t o w s e d r e g e s d n ed the p la n t a or shor t m e ll a h t t 2 . H e c o ll e c t o w t e r in g d d e la n t a n t h ey tu r n ll a shor t pe a p t o t in ey tu r ned p la n t s . t o s e e if t h w e r e t a ll . e s r t e n h t la , p e t r im or sho 6 . T h is t nt s . The re la . p ll a t t r o ll h a s e d r t hey we p la n t s a n 3. He fou nd e e t im e s a s r h t t u o b a we re or t one s . h s s a s e n o m a n y t a ll
Mendel did this over and over and always got the same results.
Gregor Mendel worked out the laws of inheritance before anyone knew genes existed.
A great discovery? Mendel told other people about his work in 1865, but no one was very interested. It wasn’t until 1900 that three scientists, working separately, re-discovered the same thing and realised how important it was. Without knowing anything about DNA, Mendel had discovered genes. 11
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Your Chromosomes and YOU Sperm cell
You’re probably a bit like each of your biological parents , but not exactly the same as them. Have you ever wondered why? You began life when one cell from your mum (an egg) and one cell from your dad (a sperm) joined together. Most cells in a human have 23 pairs of chromosomes. Sperm and eggs are special, because they each have 23 single chromosomes. This means that when they join, they make a cell with 23 pairs. So you got half of each pair from your mum and the other half from your dad. You may have noticed you’re not just one cell now… That one cell divided in two, then each of those divided over and over. You turned into a ball of cells called an embryo, then into a baby. Now you are made of billions of cells.
+
Egg cell
Fertilised egg
Embryo
Foetus
As it develops in the uterus a baby is called a foetus.
Baby
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But here’s the REALLY clever bit.
How it works Every time a cell is about to divide, it makes copies of all its chromosomes, so the new cells get a complete set. The two chromosomes that make up a pair contain genes that control the same things. So you get one copy of each gene from your dad, and the other from your mum. But, the two copies might not be exactly the same… Different forms of the same gene are called alleles . Let’s look at the gene for eye colour. We’ll only look at brown and blue eyes, to keep things simple. If both alleles say, have brown eyes, then you will have brown eyes. If both alleles say, have blue eyes, then you will have blue eyes.
m dad Ge ne fro
m mum Ge ne fro
+
e le Bro wn a ll Blu e a lle le e le Bro wn a ll
e le Bro wn a ll
+ + +
Blu e a lle le Blu e a lle le
ye C h il d ’s er u c o lo
= = =
But what happens if one allele says, have brown eyes, and the other one says, have blue eyes? Do you end up with one blue eye and one brown eye? Are your eyes blue one day and brown the next? Of course not! What usually happens is that one allele is stronger than the other, and that’s the allele that takes control. In this example, the allele for brown eyes is stronger than the allele for blue eyes.
So if you had one of each, you would end up with brown eyes.
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It’s all in your genes...
A mind-blowing introduction to the building blocks of life.