Of Charlie's age there could be no doubt. All living organisms take into their bodies known proportions of the radioactive isotopes of carbon and certain other elements. During life, an organism maintains a constant ratio of these isotopes to "normal" ones, but when it dies and intake ceases, the active isotopes are left to decay in a predictable pattern. This mechanism provides, in effect, a highly reliable clock, which begins to run at the moment of death. Analysis of the decay residues enables a reliable figure to be calculated for how long the clock has been running. Many such tests had been performed on Charlie, and all the results agreed within close limits.

Somebody had pointed out that the validity of this method rested on the assumptions that the composition of whatever Charlie ate, and the constituents of whatever atmosphere he breathed, were the same as for modern man on modern Earth. Since Charlie might not be from Earth, this assumption could not be made. It hadn't taken long, however, for this point to be settled conclusively. Although the functions of most of the devices contained in Charlie's backpack were still to be established, one assembly had been identified as an ingeniously constructed miniature nuclear power plant. The U235 fuel pellets were easily located and analysis of their decay products yielded a second, independent answer, although a less accurate one. The power unit in Charlie's backpack had been made some fifty thousand years previously. The further implication of this was that since the first set of test results was thus substantiated, it seemed to follow that in terms of air and food supply, there could have been little abnormal about Charlie's native environment.

Now, Charlie's kind, Hunt told himself, must have evolved to their human form somewhere. That this "somewhere" was either Earth or not Earth was fairly obvious, the rules of basic logic admitting no other possibility. He traced back over what he could recall of the conventional account of the evolution of terrestrial life forms and wondered if, despite the generations of painstaking effort and research that had been devoted to the subject, there might after all be more to the story than had up until then been so confidently supposed. Several thousands of millions of years was a long time by anybody's standards; was it so totally inconceivable that somewhere in all those gulfs of uncertainty, there could be enough room to lose an advanced line of human descent which had flourished and died out long before modern man began his own ascent?