Having defined life in terms of replication, it is worth considering the
darker side of replication. Although replication is a necessary and fundamental
attribute of living creatures (none of us would be alive or grow without
replication), it is also a leading cause of death and injury. For simplicity's
sake I will focus on death by replication, but we are all familiar with
ill-health by replication as you will see.
Life and Death - A Zero Sum Game
Taking into account the fact that the fixed amount of solar radiation hitting
the earth supports an estimated 200 million tons of biomass on Earth, Asimov
argued that life on Earth is a zero sum game (Asimov, 1974, p.208):
"Every time the human population increases in mass
by one ton, the mass of non-human animal life must decrease by one ton to make
room."
Take a classic predator-prey relationship which usually is taken to
mean a species of carnivore eating a species of herbivore. More tonnage of
carnivores means less tonnage of herbivores until there are too few
herbivores to feed the carnivores. Then the carnivores die off (less tonnage
of carnivores) leading to a greater tonnage of herbivores, and the cycle
continues. However the same goes for herbivory (animals eating plants,
not meat), carnivorous plants (plants eating meat) and so. Or consider a
jungle or forest cleared for farming. The native vegetation is removed
(often burned off) and the farm crop is grown in its place.
In a broad sense then replication by one population means death for another and the
two populations will not usually be of the same species.
Infectious Disease
One of the more obvious causes of death by replication has to be infectious
disease, which has so often directly affected human history on a large scale.
These diseases can be caused by the
exponential growth of populations of bacteria, viruses, fungi, or parasites.
It is due to the exponential power of these replicators - resulting in huge
populations in very short timeframes - that we have epidemics and pandemics.
Often diseases are carried and spread by rodents, flies, fleas, lice, mites,
ticks and so on. Populations of these small creatures can also explode
exponentially. Thus, the exponential growth of carrier populations can enhance
the killing power of diseases. We have exponential replication reinforcing
exponential replication to deadly effect. But without exponential growth we
would have little to fear from such diseases, and without replication there
would be no exponential growth.
In the external links I've listed some of the more deadly and well known
infectious diseases over time. Today many feel that we have won the war against
many infectious diseases, though the estimates from the Global Health Council
might suggest otherwise (Global Health Council, 2008):
"Between 14 and 17 million people die each year due
to infectious diseases – nearly all live in developing countries."
And we forget that in 1918 over 40 million people worldwide died of the Spanish Flu
virus, far more than died in World War I which preceded the pandemic (Kolata,
1999).
Even more deadly than the Spanish Flu, and thus still reasonably well known
despite occurring centuries ago, was the bubonic plague (caused by a bacterium
called Yersina pestis) which in the mid-1300s killed 60 percent of those
infected within 10 days of infection (Kiple, 1997, p.61). In what became known
as the Black Death an estimated 20 million died out of a European population of
100 million (Kiple, 1997, p.61). Less well known are the Plague of
Justinian of 542 A.D. and the many plagues that proceeded it. The Plague of
Justinian is said to have killed 10,000 a day and an estimated 25% of the
population of both Roman empires (West and East) and may have been a primary
cause of the end of Eastern Roman Empire's attempted reconquest of the Western
Roman Empire (Kiple, 1997, p.29).
Some, such as Nobel prize winner Joshua Lederberg, would rate viral
infectious diseases as the most deadly threat of all (Crawford, 2000, p.2):
"The single biggest threat to man's continued
dominance on the planet is the virus."
Smallpox is said to have caused ""...incalculable
loss of life..." in what amounted to a "....permanent
epidemic..." until successful eradication was announced by the World
Health Organisation in 1977 (Ryan, 1996, p.7). Of all the viral infectious
diseases, smallpox is generally regarded as the greatest killer of all time
(Ryan, 1996, p.7):
"When it came to plague viruses, none had ever
caused such fearful global mortality as smallpox."
Not all viral diseases have been eliminated, however. For example,
tuberculosis killed 1.5 million people in 2004 (WHO, 2004). A relatively new viral
infectious disease well known today is HIV /
AIDS. 25 million people have died from AIDS since 1981 (AVERT, 2009).
Sometimes humans harness the exponential power of a naturally occurring viral
replicator to control the population of another replicator that they consider to
be a pest. This was the case with myxomatosis, used to control rabbit
populations in Australia and elsewhere (see Rabbit
Replicators - An Exponentialist View for more). Death by replication, but
for rabbits.
But not all infectious disease has been caused by bacteria and viruses.
Ergot is a poisonous fungus that in damp conditions grows on cereals such as
rye and when ingested can affect either the central nervous system (the
convulsive form of ergotism) or cardiovascular system (the gangrenous
form) and can cause psychosis (Kiple, 1997, pp.32-33). From 591 to 1879 ergotism
- with a mean mortality rate of 41.5% - is said to have ravaged Europe 130 times
(Kiple, 1997, pp.32-33). A famous case in the USA was the Salem Witchcraft
trials, where the strange behaviour of the accused was due to ergotism and not
witchcraft (Kiple, 1997, pp.33-34). Another well known example of human
suffering caused by a type of fungus is potato blight which caused the
Irish Potato Famine in the Mid-Nineteenth century. Note that in both cases
(ergot and potato blight) it is a plant species which his infected, but with
deadly consequences for humans.
Parasites are also responsible for some deadly infectious diseases, such as
malaria and typhus. Malaria, spread by mosquito but caused by
protozoan parasites, killed 900,000 people in 2004 and debilitates many
thousands more (WHO, 2004). Typhus is spread by ticks, mites and lice but is
caused by the parasite Rickettsia. Typhus is associated with dirty and crowded
conditions and is so closely connected with famines and wars that nineteenth
century German epidemiologist August Hirsch referred to the history of typhus as
"...the history of human misery" (Kiple,
1997, p.104). The Napoleonic invasion of Russia in 1812 may well have been
forced into retreat by Typhus (Kiple, 1997, p.107). In World War I delousing
practices prevented any outbreak of typhus on the Western front, whereas
150,000 typhus deaths (including 60,000 Australian prisoners of war) occurred in
Serbia in the first 6 months of the war, deterring an Austrian attack at this
time (Kiple, 1997, p.109).
The gradual demise of various infectious diseases
is just one factor which has lead to a sharp increase in human growth rates.
Famine again
The role of a fungal mould - potato blight - is not the only example of
human famine (and death) caused by vast replicator populations. For example,
just think of the locust swarms that have existed since biblical times through
to the present day. To get an idea of the exponential replication power of a
locust population, and the sort of damage they can do, consider this quote from
the National Geographic's Locust profile (2009):
"A desert locust swarm can be 460 square miles
(1,200 square kilometers) in size and pack between 40 and 80 million locusts
into less than half a square mile (one square kilometer).
Each locust can eat its weight in plants each day,
so a swarm of such size would eat 423 million pounds (192 million kilograms) of
plants every day."
This is a classic example of Asimov's argument that life on Earth is a zero sum
game. It is also is death by replication (human famine) on a massive scale. Of
course, not all famine can be attributed to the exponential power of replicators
such as the locust or potato blight - climate plays it's role too. But death by
replication can and does occur in the form of famine.
Toxins
Toxins are produced naturally in many animals and plants. For example, some mushrooms contain deadly
toxins which can kill if ingested. However, these are only ingested by mistake
and are relatively easy to identify compared with microorganisms that may infect
or kill our food.
Toxic blooms are an example of direct death by replication due to the
exponential production of toxins. Toxic blooms can be caused by single-celled
organisms - algae, phytoplankton or bacteria ("green-blue algae" are more
correctly known as cyanobacteria). Cyanobacteria are responsible for producing
most of the oxygen in our atmosphere and they are amazing replicators (Margulis,
Sagan,1995, p.72):
"Rapidly reproducing, bacteria
properly supplied with food and water double their cells in a half hour or
faster. They have been and probably always will be the most important players
in maintaining the biosphere. A single photosynthetic blue-green bacterium
growing and dividing under ideal conditions could, in theory, produce all the
oxygen now in the atmosphere in just a few weeks."
Shell-fish feed on algae, phytoplankton and cyanobacteria and thus can
quickly accumulate toxins which are then ingested - sometimes with fatal
results - by unsuspecting humans. These toxic blooms can also kill fish directly
due to depleted oxygen levels (RCI, 2009). Death by replication, due to toxins.
"...early assembler-based replicators could
beat the most advanced modern organisms. "Plants" with
"leaves" no more efficient than today's solar cells could out-compete
real plants, crowding the biosphere with an inedible foliage. Tough, omnivorous
"bacteria" could out-compete real bacteria: they could spread like
blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter
of days. Dangerous replicators could easily be too tough, small, and rapidly
spreading to stop - at least if we made no preparation. We have trouble enough
controlling viruses and fruit flies."
Death by replication on a global scale, in a matter of days..
Cancer, in one form or another, is one of the leading causes of death in
the world today (WHO, 2004). Cancer is the result of the runaway growth of a
single cell in the body, causing a tumour (though not all tumours are
fatal, such as lipomas). As people are made up of roughly 1014
cells, and one in three people gets cancer, the odds of any one cell
turning cancerous is 1 in 300,000,000,000,000 (Crawford, 2000, p.155). But
it only takes one cell so the fact that we have so many of cells in our body
- and any one of them might malfunction - means that the odds of getting
cancer are one in three. Crawford also notes that whilst there are many
known factors that increase an individual's chances of getting cancer (such
as smoking, too much sun, or radiation), viruses are implicated in
10-20% of cancers worldwide (Crawford, 2000, p.155). It's death by (too
much) replication.
Whereas cancers represent the runaway exponential growth of a single cell,
senescence (aging) is in some sense the opposite problem, at least for
multi-cellular creatures such as humans. Single-celled creatures effectively do
not suffer aging (Ettington, 2008), but in multi-cellular creatures the number
of possible cell divisions is believed to be regulated by telomeres -
these get shorter and shorter with each cell division until no more cell
division is possible (de Magalhćes, 2003). It's death by (lack of) replication.