What a Wonderful World (2 page)

The cellular rail network enables a eukaryote to overcome one of the biggest obstacles preventing a prokaryote becoming big: getting stuff around the cell. A eukaryote, rather than having to wait for proteins to diffuse slowly through the cytoplasm, speeds them around on its rapid transit network.

But eukaryotes, despite being an enormous advance over prokaryotes, also have their limits. Orchestrating organelles is a complex activity. If a cell contained more than a few thousand of them, such orchestration would be beyond the capability of a nucleus. Eukaryotes, like prokaryotes, are a biological dead end. The way to increasing complexity lies in another direction – in cooperation on an unprecedented scale.

From the moment they arose, eukaryotes almost certainly cooperated with each other in increasingly sophisticated ways. But, about 800 million years ago, they crossed a critical threshold. Nature had put together colonies of symbiotic prokaryotes to make eukaryotes. Now it repeated the trick. It put together colonies of symbiotic eukaryotes to make multicellular organisms.

The fact that life on Earth spent about 3 billion years at the single-cell stage before it took the step to the multicellular stage is probably telling us that the step is a difficult one. This has implications for the prospects of finding extraterrestrial life.
Despite fifty years of searching, astronomers have seen no sign of intelligence elsewhere in our Galaxy. One possibility is that life is common in the Milky Way but only in the form of single-celled microorganisms.

Humans – as well as animals, plants and fungi – are all multicellular organisms. Each of us is a colony of about 100 million million cells. They come in about 230 different types, ranging from brain cells and blood cells to muscle cells and sex cells, and all are enclosed in a bag made of skin cells, no less a container than the membrane of a single cell.

Each cell has its own copy of the same DNA (apart from blood cells in their mature form, which are so utilitarian they lack even a nucleus). But whether a cell becomes a kidney cell or a pancreatic cell or a skin cell depends on the particular section of the DNA that is read, or expressed. This, in turn, depends on regulatory genes – themselves stretches of DNA – which can turn off and turn on the reading of DNA, depending on things such as the concentration of a particular chemical in the locality.

Each of the 100 million million cells that makes up a human being is a micro-world as complex as a major city, buzzing with the ceaseless activity of billions of nanomachines. It has storehouses, workshops, administrative centres and streets heaving with traffic. ‘Power plants generate the cell’s energy,’ says American journalist Peter Gwynne. ‘Factories produce proteins, vital units of chemical commerce. Complex transportation systems guide specific chemicals from point to point within the cell and beyond. Sentries at the barricades control the export and import markets, and monitor the outside world for signs of danger. Disciplined biological armies stand ready to grapple with invaders. A centralised genetic government maintains order.’
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And all of this is going on every moment of every day of our lives while we remain utterly oblivious to it. In the words of biologist and writer Adam Rutherford, ‘Each movement, every heartbeat, thought, and emotion you’ve ever had, every feeling of love or hatred, boredom, excitement, pain, frustration or joy, every time you’ve ever been drunk and then hungover, every bruise, sneeze, itch or snotty nose, every single thing you’ve ever heard, seen, smelt or tasted
is your cells communicating with each other and the rest of the Universe
.’
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We all start our lives as a single cell when a sperm, the smallest cell in the body, fuses with an ovum, the biggest cell in the body and one actually visible to the naked eye. Every human in fact spends about half an hour as a single cell before it splits into two. This is a phenomenal process in itself. In a mere thirty minutes, not only must a cell make a copy of its DNA – a process that, for speed, occurs simultaneously at multiple sites on the DNA – but it must construct something like 10 billion complex proteins. This is more than
100,000 a second
.

Within sixty minutes, the two cells split into four, then later eight, and so on. After several divisions, chemical differences across the developing embryo cause the cells to differentiate. It is a process that culminates in cells ‘knowing’ they have to be kidney cells or brain cells or skin cells. Over years, a single cell becomes a galaxy of cells – or, rather, a
thousand galaxies of cells
.

Hardly any of the cells in your body – apart from brain cells – are permanent. The cells lining the wall of the stomach are bathed in hydrochloric acid strong enough to dissolve a razor blade, so must be remade constantly. You get a new stomach lining every three or four days. Blood cells last longer but even they self-destruct after about four months. It is fair to say
that you are pretty much a new person every seven years, something that maybe explains the seven-year itch. You look at your partner and suddenly think, ‘That’s not the person I got together with!’

The cells of your body die in such prodigious numbers that, simply to replace them, you must build about 300 billion new cells every day. That is more cells than there are stars in our Galaxy. No wonder it can be tiring doing nothing.

There may be an astronomical number of cells in your body. But they are not able to carry out all the functions necessary for your survival. Not without assistance from legions of alien cells such as prokaryotes, fungi and single-celled animals called protozoans.
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In your stomach, for instance, hundreds of species of bacteria work constantly to extract nutrients from your food. If some of these ‘good’ bacteria are inadvertently killed by antibiotics, the result can be an affliction such as diarrhoea.

The alien bacteria protect you from illness by filling niches in your body that otherwise might be filled by disease-causing pathogens. The Human Microbiome Project, a five-year study funded by the US government, presented its findings in 2012. It found that the nasal passages of about 29 per cent of people contain
Staphylococcus aureus
– better known as the MRSA superbug. Since such people suffer no ill effects, the implication is that in healthy people the bugs act as good bacteria, keeping harmful pathogens at bay.

Remarkably, the Human Microbiome Project found that there are more than 10,000 species of alien cells in your body – 40 times
the number of cell types that actually belong to you. You are only 2.5 per cent human. In fact, about 5 million bacteria call every square centimetre of your skin home. The most densely populated regions are your ears, the back of the neck, the sides of the nose and your belly button. What all these alien bacteria are doing is a mystery. The Human Microbiome Project found that 77 per cent of the species in your nose, for instance, have completely unknown functions.

The sheer number of alien bacteria in your body might actually underrate their importance. The Human Microbiome Project found that microorganisms that inhabit your body have a total of at least 8 million genes, each of which codes for a protein with a specific purpose. By contrast, the human genome contains a mere 23,000 genes.
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Consequently, there are about 400 times as many microbial genes exerting their effect on your body as human genes. In a sense, you are not even as much as 2.5 per cent human – you are merely 0.25 per cent human.

Since the alien cells in your body are largely prokaryotes, which are much smaller than eukaryotes, they add up to a few kilograms or a mere 1–3 per cent of your mass. They are not encoded by your DNA but infected you after birth, via your mother’s milk or directly from the environment. They were pretty much all in place by the time you were three years old. It is fair to say that we are born 100 per cent human but die 97.5 per cent alien.

Every cell is born from another cell. ‘
Omnis cellula e cellula
’, as François-Vincent Raspall first recognised in 1825. Consequently,
every cell in our body – every cell on the Earth – can trace its ancestry in an unbroken line back to the very first cell, which appeared about 4 billion years ago. The first cell is generally referred to as the last universal common ancestor, or LUCA. Nobody knows how exactly it came about. Undoubtedly, there was a vast amount of experimentation – a huge amount of pre-evolution – before nature hit on the design.

Mistakes, or mutations, in genes accumulate at a steady rate over time. So, if one species has twice as many mutations of a particular gene as a second species, we can say it split from a common ancestor twice as far back. This is how the tree of life, first envisaged by Charles Darwin, is constructed. However, bacteria have an inconvenient habit of swapping DNA as well as passing DNA to their descendants. This means that, in the vicinity of LUCA, the tree of life is less a tree and more like an impenetrable thicket.

In physics, scientists talk of the ‘event horizon’ of a black hole – the point of no return for infalling matter. It cloaks the black hole so that nothing can be seen of its interior. Similarly, biologists talk of the biological event horizon beyond which nothing can be known. There, unfortunately, lies LUCA.

Since the time of LUCA, the Earth, despite dabbling in multicellularity, has been a bacterial world. There are believed to be something like 10,000 billion billion billion bacteria on our planet. That is a billion times more bacteria than there are stars in the observable Universe. But this might not give a true picture of terrestrial biology. Consider viruses. ‘We live in a dancing matrix of viruses,’ wrote Lewis Thomas. ‘They dart, rather like bees, from organism to organism, from plant to insect to mammal to me and back again, and into the sea, tugging along pieces of this
genome, strings of genes from that, translating grafts of DNA, passing around heredity as though at a great party.’
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Incapable of reproducing without hijacking the machinery of cells, viruses are generally not considered to be precursors of cellular life. But who knows?