Scientists claim to have discovered the smallest life forms. But are they alive?

Published: Tuesday 29 February 2000


they are believed to be the smallest forms of life. Much smaller that any known bacteria-- their size being in the realm of nanometres (or one billionth of a metre) -- they were discovered by scientists at the University of Queensland, Australia. Nanobes, as they have been named, are just 20-150 nanometres in length and are much smaller than any microorganism known so far. And if these nanobes are alive, then it is an astonishing finding. They also force us to dwell on two eternally-fascinating questions: what is life? And is there life outside Earth?
Life outside Earth Life, as we know, exists in just one place in the entire Universe -- Earth. All known living creatures are characterised by two abilities -- the ability to reproduce and the ability to carry out metabolic activity. This activity also implies the ability to assimilate simple forms of matter from the environment and convert them into complex structures. In short, to build bodies. A more fundamental description of the same two abilities can be made in slightly different words. Living creatures can transmit hereditary information (present in gigantic molecular units known as genes and housed in vehicles that effect the transmission known as bodies) from one generation to the next and can convert information-poor molecules present in the external environment into information-rich molecules in their bodies.

Everything that we think of as alive, originates from a common ancestor, maybe a set of common ancestors. Recent research has revealed that probably more than three to four billion years ago, there were a variety of living forms that could exchange hereditary material with each other. All the forms that we see today -- bacteria, fungi, plants and animals -- have evolved from the original ancestor or ancestors.

Evolution takes place whenever a chance change in the hereditary properties of a creature improves its efficiency of survival and reproduction. An obvious route to such improvement is to become bigger. There are many advantages to an increase in size: you can protect yourself against changes in the physical environment and you can make use of the economy provided by the division of labour: different parts of a large plant or animal can become specialised to do different things. Finally, you can prey on smaller creatures. In short, there is always 'room at the top'.

Nanobes also appear to be microbes. Considering that life on Earth has been around for such a long time, this must mean that microbes are extremely successful at what they do. In short, there is also 'room at the bottom'. One obvious thing that small creatures can do is to grow rapidly. We take about 20 years to reproduce but a bacterium needs only 20 minutes. The reason is that it takes much longer to build a large and complex body than a small and simple one. Indeed, when you become an efficient parasite, you lose the capacity to reproduce or metabolise on your own -- a route that viruses have adopted. Therefore, we can think of them only as 'honorary' living creatures. The problem with being small and independent is that the machinery required to reproduce and metabolise consists of extremely large molecules. One can estimate that the minimum amount of space required is approximately one thousand nanometres in each direction; perhaps this can be pushed down to a few hundred nanometres, but not much less.

In 1998, scientists at the National Aeronautical and Space Administration ( nasa) reported the existence of what they thought were fossil nanobacteria in a Martian meteorite, a claim that has generated a great deal of controversy and scepticism. Till now, there have been no reports of anything like Martian nanobacteria seen in fossils on Earth, which has also added to the scepticism. Nanobes are roughly the same size as Martian nanobacteria, but otherwise quite different. Are they alive? This depends on how one looks at them. They seem to grow spontaneously, contain deoxyribonucleic acid ( dna) and are enriched in carbon, nitrogen and oxygen. That would seem to clinch the issue. But what sort of dna? How is their dna related to other known microorganisms? We need to wait and find out whether the dna is not a contaminant.

It is also a great mystery how nanobes manage with such small bodies? Clearly they are not large enough to house all what we think of as the most basic constituents of a living cell. But it might be that each nanobe contains only part of what is needed, with a collection of nanobes being the functional equivalent of a single living cell? If so, these nanobes display an unprecedented degree of cooperative social behaviour.

To sum up, we still lack conclusive evidence for the occurrence of reproduction and metabolism in nanobes. Nor, even assuming that they are living creatures, do we have a plausible hypothesis regarding their evolutionary origin. But, until we do, the possibilities remain fascinating.

The author is with the Indian Institute of Science, Bangalore

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