If humans became extinct, how long would it take for all traces of us to vanish?

Stonehenge is at least 4,000 years old and still visible today, old and still visible today, and monuments buried nearby could be even older. Most modern buildings aren’t that robust, but some traces would likely remain for at least 10,000 years, even if it was just the magnetic trace of the steel bars inside concrete blocks. When Hong Kong airport was constructed in the 1990s, the island of Chek Lap Kok was flattened and extended, and the straight edge of its northern coastline will be a clue to our civilization for tens of thousands of years. Our atmosphere also has high level of plutonium-239 due to nuclear weapons testing during cold war. This isotope only occurs in nature in incredibly small amounts, and will be detectable as a pollutant for at least 250,000 years.

But the most enduring signs of civilization will probably be deep mines in hard rock, such as South African gold mines Australian lead mines. Here, visiting aliens would be able to see signs of our civilization for millions of years, as the tunnels fill up with sediment washed down by rainwater to create massive industrial ‘fossils’.

WHAT  IS  THE  MAXIMUM  NUMBER  OF  NAMES  A  PERSON  CAN  REMEMBER ?

There’s no known limit! If you ask a mnemonist or memory savant to learn a list of names they may remember thousands, tens of thousands or even hundreds of thousands with no trouble, just as they can learn lists of thousands of digits. Some people who have a neurological condition called ‘hyperthymesia’, remember everything that happens to them every day, including the name of every person they have ever met.

The rest of us evolved to cope with no more than about 150 social relationships. This is known as Dunbar’s number after the anthropologist Robin Dunbar. He discovered that groups of hunter-gatherers, units in armies, divisions in businesses and many other groups tend towards a limit of 150. And it seems that social media do not change our basic nature. Even people who have thousands of ‘friends’ on Facebook rarely maintain more than 150 meaningful relationships.