It seems to be a combination of raw energy rundown and storage capacity overload. This can be accompanied by mild nausea brought on by a rapidly changing itinerary of people and topics, electronic and paper information transfer. I have reached my limit - and it is principally mental.

All biological organisms function as information processors; they take it in, process it and use it to locate the necessary energy sources for survival. The more efficiently organisms do this, the more successfully they and their offspring continue their existence. Organisms are perpetuated at the expense of the less efficient - the smartest win.

Billions of years of life have been driven by random mutation and natural selection acting on carbon-based molecular systems. Homo sapiens arose over the past two million years of this process. Our future evolution depends on understanding that living creatures are information processors; that is, consumers of entropy rather than just energy. This implies that systems more efficient at information processing may supplant us.

Indeed, for task-specific applications this is axiomatic and exemplified by auto-pilots, engine management systems, automated factories and mass production plants.

But perhaps the difficulties the world chess champion, Garry Kasparov, had with Deep Blue give a better barometer of progress. Only 10 years ago many believed such open-ended games as chess would always be beyond computers. No longer. Given the rate of progress in computer hardware and artificial intelligence, Deep Blue will have Kasparov in even deeper trouble within five years.

The ability of an organism, or organisation, to process information about its environment is a driving force behind evolution. If there is evolutionary pressure to evolve better brains to survive, then genetic engineering and other biological options will not help if our brain is inherently limited by architecture and operational modes.

The next step in evolution would then be to appropriate silicon as the intelligence medium to augment our wetware. Future evolution would then be driven by mechanisms and forces radically different to those manifestly of nature, giving rise to a creeping carbon-silicon mix.

At some point, biological systems become inherently limited as they encounter fundamental physical limitations that constrain, direct or prevent further evolution in some direction.

The most obvious examples are the limitations on size imposed on insects by their ability to transport oxygen; or the stress limits of bone in land-based mammals dictating the leg thickness needed to support their weight. For us, it is the limitations to both our frame and brain. Having enjoyed two million years of encephalisation, we have hit the end stop - our brain gets no bigger.

If we as a species are to compete in a machine-dominated culture, where are we going to find the necessary brain capacity? For certain it will not be through the enhancement of our carbon-based wetware. We have two choices: internal or external silicon extension.

Peter Cochrane holds the Collier Chair for the Public Understanding of Science & Technology at the University of Bristol. His home page is:
http://cochrane.org.uk