Travel Reference
In-Depth Information
much energy from a sugar molecule as those that cannot. Luckily for us, the
invasion of our cells by the bacteria gave our ancestors back that ability.
Why were our ancestors so careless as to lose the ability to burn food
with the aid of oxygen in the fi rst place? Perhaps there was an evolutionary
trade-of involved. In order to respire ei ciently and gain the maximum from
their food, single-celled organisms need special cell membranes that can
generate a strong electric charge dif erence between their inner and outer
surfaces. If there are breaks in the membranes they will lose the charge dif er-
ence, just as batteries do when they short out. These cells are unable to wrap
themselves around and engulf solid pieces of food without making breaks in
their membranes thus destroying this essential charge dif erence.
So it may be that our ancestors made a Faustian bargain: they lost the
ability to burn food using oxygen, but in exchange they were now able to
modify their membranes so that they could engulf smaller organisms like
tiny bacteria and algae.
Although they were now able to eat other creatures, becoming the world's
fi rst predators, they could not extract much food energy from their victims.
Luckily, this downside to their Faustian bargain was short-lived. When the
bacteria that would eventually become mitochondria invaded the cells of
our carnivorous ancestors, their ability to carry out aerobic respiration was
restored. And our ancestors did not have to lose the ability to eat solid food,
because all of this benefi cial respiration was now taking place deep inside
their cells instead of on the surface. The way was open for our ancestors to
become highly ef ective predators. They were able not only to catch other
creatures but also to extract lots of energy from them. Our ancestors became
the tigers of that simple world.
The respiring bacteria soon became permanent fi xtures in their new
hosts, entering into a relationship with them called
endosymbiosis
. Our ances-
tors nurtured the bacteria carefully, passing them like family jewels from one
generation of host cells to the next.
The mitochondrial descendants of those bacteria now live in immense
numbers in most of the cells of our bodies. They are cute little ovoid struc-
tures, still looking a bit like bacteria. As they pump energy-rich compounds
into our systems, they keep our metabolisms running at full blast.
