Download 252 Humans still evolving

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Number 252
Is human evolution finally over?
Organisms adapt to their environment. The process of natural
selection weeds out individuals that are less well adapted. Those
that have useful adaptations are more likely to survive, breed and
thus pass on their genes. Thus organisms change or evolve via the
process of natural selection.
Traits that appear to be still evolving include lactose tolerance, skin
colour, hair formation, the metabolism of foreign compounds and,
oddly enough, myopia (Table 1)
Table 1. Traits arising from recent human evolution
But it could be argued that humans don’t need to adapt – they have
the technological ability to manipulate the natural world to suit
themselves and reproductive techniques can make the infertile,
fertile. Our survival is more dependent on our technologies than
our genes isn’t it? So the question is: Have humans stopped
evolving?
Before considering this, have a think about what, if any, selective
forces still affect us.
Chimpanzees are our closest relatives. Humans split from
chimpanzees about 8 million years ago. Both the human and
chimpanzee genome have now been published and geneticists are
busy looking at how our genome has changed since we split. New
genes have clearly emerged in recent human history. Scientists at
Chicago University recently discovered two new genes involved in
brain development. The first arose between 14000-60000 years ago
and is now carried by about 70% of the human population. The
second gene is even more recent; it emerged between 500-14000
years ago and is found in about 25% of the human population. So
far, we don’t know precisely what these genes do but one theory is
that they favour the development of larger brains – which could
clearly offer a selective advantage.
Trait
Selective pressure
Myopia (short-sightedness)
Invention of spectacles
Proportion of population able
to digest lactose (milk sugar)
Invention of dairy farming
Increasing proportion of
population in some parts of
Africa that possess the CCR532 gene. This gene confers
some protection against HIV-1
AIDS
Pale skin colour
Movement of humans from
Africa to Europe 45,000 years
ago
Case Study: Lactose tolerance
Some people are unable to produce the enzyme lactase, the enzyme
needed to break down lactose, one of the sugars in milk. These
individuals are described as lactose intolerant. Lactose intolerance
varies greatly internationally (Table 2).
Table 2. Lactose intolerance
HapMap project
Population
This is an international study to identify genetic similarities and
differences in humans. Four populations have been analysed in
detail:
1. the Yoruba of Nigeria
2. Han Chinese from Beijing
3. Japanese from Tokyo
4. US citizens of Europeans ancestry
Asia
95
Afro-Caribbean
75
Mediterranean
50
Northern European
10
% of population unable to digest lactose
In fact, most adults around the world lose the ability to produce
lactase as they get older. But 90% of Europeans possess a lactaseproducing allele that remains active into adulthood.
From this project and many others that have interrogated its data, it
is clear that humans are still evolving and different genes have
been selected for in each of these four populations.
Recent research has shown that there are four different gene
mutations correlated with lactose tolerance around the world. These
four mutations appear to be completely independent of one another
- strong evidence of convergent evolution.
Researchers have found that about 1800 genes are experiencing
positive selection. Many of these appear to be linked to disease
resistance, diet and skeletal development. Such data is being used
in conjunction with anthropological data and suggests that changes
to agriculture, in particular, dramatically influenced human evolution.
These four mutations are recent – within the last 7000 years or so.
Analysis of DNA from bone samples from Neolithic Europeans
(dated between 5840 BC and 5000 BC) revealed that none of them
had the adult lactase allele. The populations that possess the lactose
tolerance mutations are those that started domesticating cattle. As
humans began using cows milk as a food, those that possessed the
lactose tolerance allele had a selective advantage, which translated
into more offspring than those who were intolerant.
By analysing some of the estimated 10 million regions of the genome
where single base errors have occurred – single nucleotide
polymorphisms (SNPs) – scientists have show that many new alleles
have evolved over the past 10 000 years.
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252 Is human evolution finally over?
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Our changing genes
The human genome is constantly changing as a result of random mutations. Some scientists argue that this in itself is human evolution.
Some of these mutations may confer a selective advantage. Given the right selective pressures, this would result in human evolution.
Since human population is increasing rapidly, so too is the size of our gene pool. Some scientists believe that changing human behaviour
means that any advantageous mutations may spread through the population faster than ever. The argument is that people are far more
likely to meet and reproduce with other people who are like them e.g. of similar intelligence, similar personalities and mental health. This is
known as assortative mating and it may well be becoming more efficient as a result of increased access to higher eduction, internet dating
etc.
Controversially, some scientists predict that as a result of assortative mating, genetic screening and manipulation, humans will inevitably
become beautiful, symmetrical and intelligent.
Case Study: Skin colour
There is a mass of evidence that all humans evolved from a common ancestor in tropical Africa. We spread out across the world.
Different populations faced different selection pressures and evolved in different ways. Skin colour is one example. Populations living
close to the equator for many generations have darker skin colour than those living further from the equator.
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Darker skin (melanin) prevents UV light from penetrating the skin
Excess penetration of UV destroys folic acid
Folic acid deficiency during pregnancy can lead to seriously handicapped babies
Insufficient penetration of UV prevents formation of vitamin D
Vitamin D deficiency leads to rickets
So, near the equator, where UV is most intense, mutations that led to dark skin might also lead to healthier babies that, in turn, were
more likely to reproduce and pass the gene for dark skin on. For the populations that migrated away from the equator and out of Africa
lighter skin was an advantage because the UV wasn’t a danger and they were less likely to develop rickets.
Summary
So humans are still evolving and different human populations are evolving in different ways at different speeds. This raises serious ethical
problems – could it be argued that some populations have evolved further than others? Does this mean that they are in some way “better”
or “fitter” human beings, in some way better adapted to surviving? Hitler made such arguments!
Practice Questions
1. The diagram shows a suggested relationship between modern
humans (Homo sapiens), some other, now extinct human-like
(Homo) species and chimpanzees. The figures refer to estimated
brain size.
(a) What is the major source of the data in the diagram?
(1)
(b) Use your knowledge of natural selection to suggest
an explanation of the changes to brain size shown (4)
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3
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Australopithecus
450 cm3
Homo habilis
550 cm3
Homo erectus
900 cm3
Homo
neanderthalensis
1500 cm3
Homo sapiens
1400 cm3
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1. (a) Fossils;
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Answers
millions
of years
ago
Chimpanzee
400 cm3
(b) development of new species over time
trend is increasing brain size in Homo species;
extinction of smaller brained species;
bigger brain gives selection advantage/ greater
intelligence;
e.g. tool use;
more likely to survive and reproduce;
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2. AIDS is a disease that kills many young people in Africa.
Suggest how AIDS may be seen as an evolutionary force
that still influences humans in Africa (3).
2. AIDS kills many before they reach child-bearing age;
children with genetic resistance to HIV may survive;
survivors may reproduce and pass on the gene;
resistant gene spreads through population;
number indicates
average brain size
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Acknowledgements:
This Factsheet was researched and written by Kevin Byrne.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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