The drive to explore has taken humans to most ofthe habitable planetMarco Bottigelli/Getty ImagesThe Explorers GeneAlex Hutchinson (Mariner Books (UK, 10 April; US, on sale now))Approximately 50,000 years ago, our ancestors the first modern humans set out from their African homeland in droves. We dont know for sure what prompted this mass uprooting (sometimes known as the Great Human Expansion), but our species staggering geographic spread is proof of its success.In relatively short order, humans made it to more or less every habitable corner of the planet, writes Alex Hutchinson, a journalist,