Different types of cells have different lifespans, e.g.: 

• we shed our skin cells about every 35 days

• red blood cells live about 120 days, platelets 6 days and white cells less than a day

• most of the adult skeleton is replaced about every 10 years

• the average age of a fat cell seems to be about 10 years

• a 25-year-old heart replaces about 1% of all its cardiomyocytes (heart muscle cells ) over the course of a year, while a 75-year-old heart replaces about half a percent

• our neocortical neurons, the cell type that mediates much of our cognition, are produced prenatally and retained for our entire lifespan

Think of an experience from your childhood. Something you remember clearly, something you can see, feel, maybe even smell, as if you were really there. After all, you really were there at the time, weren’t you? How else would you remember it?

But here is the bombshell: you weren’t there.

Not a single atom that is in your body today was there when that event took place … Matter flows from place to place and momentarily comes together to be you.

Whatever you are, therefore, you are not the stuff of which you are made.

If that doesn’t make the hair stand up on the back of your neck, read it again until it does, because it is important.

The ship wherein Theseus and the youth of Athens returned [from Crete] had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same.

Nature has rolled the dice trillions and trillions of times and has learned to pick diversity as the best long-term bet. It would have been far less complicated to go with one species, but nature has consistently been willing to pay a hefty price to keep its options open. You never know what’s coming down the pike and which genetic potential will be most needed to meet the next challenge.

While we now know that Turing was too optimistic on the timeline, AI's inexorable progress over the past 50 years suggests that Herbert Simon was right when he wrote in 1956 "machines will be capable ... of doing any work a man can do." I do not expect this to happen in the very near future, but I do believe that by 2045 machines will be able to do if not any work that humans can do, then a very significant fraction of the work that humans can do. Bill Joy's question deserves therefore not to be ignored: Does the future need us? By this I mean to ask, if machines are capable of doing almost any work humans can do, what will humans do? I have been getting various answers to this question, but I find none satisfying.

A typical answer to my raising this question is to tell me that I am a Luddite. (Luddism is defined as distrust or fear of the inevitable changes brought about by new technology.) This is an ad hominem attack that does not deserve a serious answer.

We are facing the prospect of being completely out-competed by our own creations. A more thoughtful answer is that technology has been destroying jobs since the start of the Industrial Revolution, yet new jobs are continually created. The AI Revolution, however, is different than the Industrial Revolution. In the 19th century machines competed with human brawn. Now machines are competing with human brain. Robots combine brain and brawn. We are facing the prospect of being completely out-competed by our own creations. Another typical answer is that if machines will do all of our work, then we will be free to pursue leisure activities. The economist John Maynard Keynes addressed this issue already in 1930, when he wrote, "The increase of technical efficiency has been taking place faster than we can deal with the problem of labour absorption." Keynes imagined 2030 as a time in which most people worked only 15 hours a week, and would occupy themselves mostly with leisure activities.

I do not find this to be a promising future. First, if machines can do almost all of our work, then it is not clear that even 15 weekly hours of work will be required. Second, I do not find the prospect of leisure-filled life appealing. I believe that work is essential to human well-being. Third, our economic system would have to undergo a radical restructuring to enable billions of people to live lives of leisure. Unemployment rate in the US is currently under 9 percent and is considered to be a huge problem.

Finally, people tell me that my concerns apply only to a future that is so far away that we need not worry about it. I find this answer to be unacceptable. 2045 is merely a generation away from us. We cannot shirk responsibility from concerns for the welfare of the next generation.

Here’s a current example of the challenge we face... At the height of its power, the photography company Kodak employed more than 140,000 people and was worth $28 billion. They even invented the first digital camera. But today Kodak is bankrupt, and the new face of digital photography has become Instagram. When Instagram was sold to Facebook for a billion dollars in 2012, it employed only 13 people. Where did all those jobs disappear? And what happened to the wealth that all those middle-class jobs created?

evolution doesn't always proceed steadily

The following is an example of a 12-station HICT program. All exercises can be done with body weight and implements easily acquired in almost any setting (e.g., home, office, hotel room, etc.). The exercise order allows for a total body exercise to significantly increase the heart rate while the lower, upper, and core exercises function to maintain the increased heart rate while developing strength.

Exercises are performed for 30 seconds, with 10 seconds of transition time between bouts. Total time for the entire circuit workout is approximately 7 minutes. The circuit can be repeated 2 to 3 times.

1. Jumping jacks Total body

2. Wall sit Lower body

3. Push-up Upper body

4. Abdominal crunch Core

5. Step-up onto chair Total body

6. Squat Lower body

7. Triceps dip on chair Upper body

8. Plank Core

9. High knees/running in place Total body

10. Lunge Lower body

11. Push-up and rotation Upper body

12. Side plank Core

To land a spacecraft on Europa, with the heavy equipment needed to penetrate the ice and explore the ocean directly, would be a formidable undertaking. A direct search for life in Europa's ocean would today be prohibitively expensive. But just as asteroid and comet impacts on Mars have given us an easier way to look for evidence of life on that planet, impacts on Europa give us an easier way to look for evidence of life there. Every time a major impact occurs on Europa, a vast quantity of water is splashed from the ocean into the space around Jupiter. Some of the water evaporates, and some condenses into snow. Creatures living in the water far enough from the impact have a chance of being splashed intact into space and quickly freeze-dried. Therefore, an easy way to look for evidence of life in Europa's ocean is to look for freeze-dried fish in the ring of space debris orbiting Jupiter. Sending a spacecraft to visit and survey Jupiter's ring would be far less expensive than sending a submarine to visit and survey Europa's ocean. Even if we did not find freeze-dried fish in Jupiter's ring, we might find other surprises -- freeze-dried seaweed, or a freeze-dried sea monster.

Freeze-dried fish orbiting Jupiter is a fanciful notion, but nature in the biological realm has a tendency to be fanciful. Nature is usually more imaginative than we are. Nobody in Europe ever imagined a bird of paradise or a duck-billed platypus before it was discovered by explorers. Even after the platypus was discovered and a specimen brought to London, several learned experts declared it to be a fake. Many of nature's most beautiful creations might be dismissed as wildly improbable if they were not known to exist. When we are exploring the universe and looking for evidence of life, either we may look for things that are probable but hard to detect or we may look for things that are improbable but easy to detect. In deciding what to look for, detectability is at least as useful a criterion as probability. Primitive organisms such as bacteria and algae hidden underground may be more probable, but freeze-dried fish in orbit are more detectable. To have the best chance of success, we should keep our eyes open for all possibilities.

Many species of terrestrial plants, including the skunk cabbage that sprouts in February in the woods of Princeton, New Jersey, where I live, are warm-blooded to a limited extent. For about two weeks the skunk cabbage maintains a warm temperature by rapidly metabolizing starch stored inside the part of its anatomy known as the spadix, which contains the hidden flowers with their male and female structures. According to folklore, the spadix is warm enough to melt snow around it. The evolutionary advantage of warm-bloodedness to the plant is probably that it attracts small beetles or other insects that linger in the spadix and pollinate the flowers. The spadix is not a greenhouse, and the supply of starch is not sufficient to maintain a warm temperature year-round. No terrestrial plants are able to stay warm through an Arctic winter. On Earth polar bears can flourish in colder climates than trees can. It seems to be an accident of history that warm-blooded animals evolved on Earth to colonize cold climates, whereas warm-blooded plants did not. On Mars plants might have been pushed to yet more drastic adaptations.

Plants could grow greenhouses (so far the idea remains a theory) just as turtles grow shells and polar bears grow fur and polyps build coral reefs in tropical seas. These plants could keep warm by the light from a distant Sun and conserve the oxygen that they produce by photosynthesis. The greenhouse would consist of a thick skin providing thermal insulation, with small transparent windows to admit sunlight. Outside the skin would be an array of simple lenses, focusing sunlight through the windows into the interior. The windows would have to be small, to limit the loss of heat from outward radiation. The plant would also need deep roots, to tap water and nutrients from warmer layers underground. Inside the greenhouse the plant could grow leaves and flowers in an oxygen-containing habitat where aerobic microbes and animals might also live. Groups of greenhouses could grow together to form extended habitats for other species of plants and animals. An attendant community of microbes and fungi might help the plants to extract nutrients from the local ice or soil. Pores in the outer skin of the greenhouse might open to admit carbon dioxide from the atmosphere outside, with miniature airlocks and cold traps to keep losses of oxygen and water to a minimum.