Transcript Powerpoint for Lecture, 15 September

The Pre-History of
Human Disease
McNeill, Chs. 1-2
Crawford, Chs. 1-3
Co-evolution of pathogens and humans
• One of the challenges of studying the history of disease is that both
pathogens and human beings are products of evolution, and
evolutionary changes in one can have an impact on the other
• Some scientists call this “the evolutionary arms race” – and warn that
microbes are getting the upper hand (explained today by the creation
of superbugs that have developed immunity to antibiotics)
• Another challenge is that we do not have a lot of direct evidence
(microbes rarely leave fossils) of the early relationship between
microbes and humans – so scientists have to infer that relationship
from the results
Human vs bacterial evolution
• Bacteria and humans are not only evolving together, but they have
their own individual evolutionary processes
• Bacteria evolve at a much faster rate than humans – because they
reproduce more easily and abundantly
• Bacteria also produce many more genetic mutations than humans,
and those genetic mutations are passed on at a faster rate
• In both cases, the goal is survival of the species, and both develop
advantages that help them to do that
Gut microbes:
we can’t function without them
• The human gut contains about 1 kg of bacteria – which together
weigh about as much as the human brain
• Scientists today are just beginning to understand the function of
these microbes, and how those bacteria influence the human body
• Many scientists have shown that these bacteria actually have a very
strong influence on human behaviour and cognitive function
• Thus when those bacteria are not doing their job properly, this can
have an effect on mental health – a testament to the long and
complicated relationship between microbes and humans
Immunity
• In the process of co-evolution, humans have built up immunity to
many microbes, and in turn microbes have decreased their virulence
to keep their hosts healthy
• There is an innate or non-specific immune system, which fight
infectious pathogens in a generic way
• There is also an adaptive or acquired immune system – which is
specific to a certain pathogen – for example someone who is exposed
to measles will be protected from measles for life; some others, like
chickenpox, do not provide lifetime protection
Ecological balance vs catastrophe theory
• Some scientists posit that nature seeks to balance things out: when
one change happens that disrupt the balance of nature, nature will
correct itself with a change that will balance things out – this is
McNeill’s position in Plagues and Peoples
• Most scientists have now discredited this theory, and instead
subscribe to a dynamic or chaotic view of nature
• Regardless of this debate, however, it does appear that early humans
did not suffer greatly from infectious disease – and it appears for the
most part to have been a harmonious and mutually beneficial
arrangement
Cultural evolution vs biological evolution
• One advantage that humans have is is that we can transmit things to
future generations by other means that our genes, by using language,
(and eventually writing), and with demonstrations using other senses
• In other words, this means that some of the cultural advances in
humans, like art for example, move the species forward in a similar
fashion to biological advances
The earliest humans
• The human lineage originated about 6 million year ago
• By 3.5 million years ago there is evidence of bipedalism
• By 2.5 million years ago there is evidence of stone tool making
• 500,000-700,000 years ago Homo erectus migrated out of Africa
• 200,000 years ago Neanderthals
• The most accepted theory is that homo sapiens evolved from a a
single origin, in Africa, just over 100,00 years ago
Migration of Homo Sapiens
By NordNordWest - File:Spreading homo sapiens ru.svg by
Urutseg, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=34697001
From trees to savannahs
• Once hominids developed bipedalism they could leave the trees
behind and look for life on the savannah in Africa
• Here they could face drier and colder clime than the forest, which
meant that they could leave behind the more tropical diseases of the
rainforest that required the heat and moisture of that eco-system to
survive
• It was in this stage that they made advances in tool-making that
allowed them to hunt small game and to gather food – hence the
common name hunter-gatherer societies
European early modern humans –
Cro-Magnon
• The famous cave paintings in Lascaux
have been dated to about 17,000 years
ago
• The paintings depict that they are big
game hunters – using spears, javelins,
and spear throwers
• They used stone tools, wove baskets and
sewed garments made out of animal
hides and furs
Famous cave painting from
Lascaux in Southwest France
Disease and hunter-gatherer societies
• Hunter-gatherer societies were affected by a variety of infectious and
parasitic diseases, but the small size of communities for the most part
kept them safe from epidemics
• Researches have show that malaria affected these societies, but again
their small size, and their distance from the rainforest and the habitat
of the tsetse fly that carries malaria kept them from succumbing too
badly to that
• Furthermore, their varied diet helped to buffer them from infectious
diseases
The Neolithic Revolution c. 10,000 BC
• The so-called Neolithic revolution is important because it reflected a
fundamental change in the way people lived – moving from a huntergather type of subsistence to one based on agriculture and
permanent settlements
• One of the things that was pushing the agricultural revolution was
availability of food; many of the large game species became extinct,
largely because they had been overhunted
• Another factor was climate change – a warmer and drier climate
made it difficult to hunt and forage for food
New food crops introduced around the world
Disease consequences of sedentary life
• A growth both in population and population density, given more
opportunity for pathogens to spread
• Domestication of plants and animals led to the introduction of new
pathogens into the human population
• Sedentary lifestyle meant more human waste, which were a breeding
ground for pathogens and an opportunity for them to spread to the
human population
• Increased growth of lice and fleas introduced new disease vectors
• Measles, smallpox, plague and tuberculosis became widespread
under these new conditions
A look ahead
• In Tuesday’s seminar we will discuss Ann Carmichael’s article
“Infectious Disease and Human Agency: An Historical Overview”
which gives a broad view of how humans are doing in the figt against
infectious disease
• Next Thursday’s lecture will look more closely at the impact of disease
on ancient civilizations