Senescence

The first person to live to be a thousand years old will have been specifically bio-engineered for such longevity, clonally descended (with intentional genetic modification) from other predecessors who were also bio-engineered to increase their longevity.

I think one of the reasons for the popularity of Aubrey de Grey's nigh-alchemistic views of aging is that they are phrased in such a way to suggest that disagreeing with them sounds anti-scientific (or at the very least, skeptical of the potential for scientific progress). The purpose of my claim above is to provide an equally trans-humanist view, that is equally based on the belief that people will eventually find ways to better overcome senescence, without making De Grey's mistake of being utterly and completely wrong.

Because de Grey is wrong. (Dead wrong, one might say.)

The mainstream academia position on de Grey is that his claims deal with research topics that are too poorly understood to provide proof of the falsenesss of de Grey's claims, but the available evidence strongly indicates that he is wrong, and once more is known, his position will be completely falsified. That's fair. Most of the evidence that strongly refutes de Grey's position is soft evidence. Nobody understands precisely how aging works but all of the theories that explain anything at all are incompatible with de Grey's beliefs. But you can't really use any of them to refute them because all of them have enough problems of their own.

But the soft evidence against him is overwhelming.

First of all, humans already have the longest average life span of any endotherm and the largest average lifespan of any terrestrial animal, even if you limit your sample to organisms that survive to adulthood in order to give reptiles a fighting chance at high average lifespans since they have worse than astronomical mortality rates shortly after hatching. Humans also have the highest recorded maximum lifespan of any endotherm.

Terrestrial animals are a worthwhile category because living on land is a lot harder than living in water. Read Margulis if you want a little bit fuller explanation. More or less, earth-born life is aquatic, and terrestrial life survives by internally preserving oceanic conditions. Moreover, all animals start their life in a liquid environment (of an egg, womb, or body of water). Instead of adapting a way for zygotes to survive outside of an aquatic environment, mammals have evolved a way to produce an artificial aquatic environment inside the females of the species, even as some mammals also evolved a dry environment for their young to mature for a while after birth. If you can only find aquatic animals to support some claims of what is possible, you probably aren't making claims that apply to longevity. Evolving to live on land forced terrestrial animals through a very tight bottleneck, which mercilessly selected in favor of traits needed to perpetuate a species on land without any regard for what traits members of those species might later wish they had had preserved. I think chordates traded off the potential for biological immortality long before adapting to living on land, but at any rate, the likelihood of them having preserved it past living on land is infinitesimal. There was just too much of an optimization for something else in particular.

The endothermy argument is something else altogether. We tend to measure lifespans in years. Years are biologically irrelevant. If we wanted to measure lifespans in something more biologically relevant we should focus on metabolic turnover and cell divisions. Both of these are way higher in endotherms (mammals, birds, and tuna!) than they are in ectotherms (non-endotherms). Giant tortoises spend much of their lives biologically dead. They hibernate so thoroughly that they don't even have a heart beat. Since they don't have to maintain their body temperature in their cells, their cells don't have to do anything. When it gets too cold, they just die for a little while and wait for the sun to come back out. True story. A giant tortoise living to be three hundred is biologically equivalent to a mammal living to be about four. (Not four hundred, just four.)

Then there's complexity. Talking about the simplicity or complexity of organism is extremely out of vogue. But fashion is dumb anyways, so I don't care. Biologically immortal organisms are extremely simple. Vascular plants have three organ-types (for want of a better term). They have leaves, stems, and roots. Flowers are a type of leaf. Tubers are a type of stem. Bulbs are a type of leaf. Etc. This isn't semantics or pedantry. Different organ-types are made up of different tissue-types which are made up of different cell-types. Different cell-types rely on different conditions from each other to survive. Plants only have a few dozen cell types, so they don't have to code for preserving that many different modes of survival. Biologically immortal organisms, whether they are plants or animals, are ridiculously simple as measured in the number of different cell types that they have. For reference, you have more types of tissue that are unique to your ear than a clam has in its whole body. By unique to your ear, I mean specialized tissue types that only occur in your ear and don't occur anywhere else in your body. Your ear isn't even close to being the most complicated organ-like-thing in your body either. (That would be your brain, in case you're wondering. There are more cell types in the brain than there are tissue types in the ear, and there are so many different tissue-types in the brain that, to the best of my knowledge, no one has attempted to make an exhaustive list. As of 2014, the authors of the leading neuroscience textbook did not believe that an exhaustive list of neurotransmitters is known yet, and that list is tiny by comparison to trying to exhaustively identify all of the different kinds of tissues that occur in the brain. Brains are ridiculously complicated, just in terms of their raw hardware before you begin to look at the way that that hardware is organized. In a very overtly evolved sort of way. We're talking tangled, haphazard, messy, disorganized complexity.)

And then there's the data. If sequoias died at a young enough age, we'd think that they were biologically immortal. For the first thousand years of their lives, they exhibit all of the traits of biological immortality. Their chances of living another year increase with every successive year, etc. (Actually, this general trait of having once chance of living another day increase with every successive day is true of most plants for the first few years of their lives. Hell, it's true of most animals too during their early life. Mortality rates tend to be pretty high for the young of most species.) If sequoia had a high enough mortality rate that they all died before living to be 1500 years old we'd think they were biologically immortal. But they don't. They live to be about five times as old as the oldest biologically immortal organism, and they only begin to exhibit senescence once their a thousand years older than the oldest known biologically immortal organism. Sequoia are "biologically immortal" by the way that characteristic is defined for a far longer period of time than any organism that is actually believed to be biologically immortal. Most "biologically immortal" organisms have very short life spans. Sponges, clams, and sea urchins are the exceptions. They sometimes live to be a few hundred years old, and they have members that exhibit decreased mortality with age for that whole lifespan. (Also, PETA claimed that one particular lobster was over 100 years old in a bid to save its life, but their calculation was based off of some very spurious assumptions. Moreover, lobsters are known not to be biologically immortal, since their chances of dying while molting [or simply becoming unable to molt] increase dramatically as they age. Becoming unable to molt is eventually fatal for a lobster.) Hydra can live to be up to four years old without showing any increase in mortality as they age! Whoop-dee-doo! Humans can also live to be up to four years old without any increase in mortality over that period... while being much more biologically active.

So at this point, we've got four major strikes against the idea of a human living to be a thousand. Simple organism (as measured in the number of cell-types, tissue-types, or organ-types the organism possesses) live to have a lot longer lifespans than more complex organism. Aquatic animals are able to have much longer lifespan than terrestrial animals. Ectotherms have a much longer lifespan as measured in years than endotherms but a much shorter lifespan as measured by biologically-relevant criteria such as total metabolic activity or cell divisions. And when we look at extremely simple, aquatic, endotherms, we find that they still have maximum lifespans capped at a few hundred years. The oldest clam ever discovered was 507 years old. The most longevic plants can live to perhaps 5,000 years old, and some scientists believe that a few sponges might be 10,000-20,000 years old, but at this point we're talking about things that are ridiculously simple compared to amniotes.

The oldest known terrestrial homotherm died on August 4th 1997. She was 122 years old. Her name was Jeanne Calment, and she was human. No other mammal is known to have lived that long. (Though bowhead whales [not a terrestrial animal] are thought to live up to 150 years, possibly longer.) No bird is known or thought to have lived that long.

When somebody says that senescence isn't biologically inevitable in the context of talking about extending human life spans, that person is saying something pretty ignorant. The available evidence strongly suggests that humans are already flirting with the upper bound of what's biologically possible for organisms with our biological history.

But what about science? Technology! Progress! Onward! Yay science! Hurrah! Hurrah! Hurrah!

Isn't the fact that humans live longer than any other terrestrial homotherms evidence in favor of modern medicine?

No. Not at all.

People from parts of the world with no modern medicine have been reliably shown to be 110-115 years old. Any alleged medical progress that humanity has made in the past four hundred years might have extended the upper bound on human lifespan by about five years. Probably not. There are a lot of other things like adequate nutrition and improved hygiene that ought to increase longevity in the first world relative to the third world.

I'm not knocking medicine, in general. Claims that medicine has improved the average human lifespan are very substantiated. Childbirth used to be a major cause of death for women. Medical advances of the past century and a half have practically eliminated deaths due to childbirth. Childhood mortality due to contagious disease also used to be a major cause of death, and vaccines have pretty much eliminated those. So I'm not knocking medicine altogether. I'm strongly in favor of the medical position in two of the more controversial issues in medicine. But I see no evidence whatsoever to suggest that medical progress has done anything to begin overcoming senescence. Maybe, modern medicine has extended them by 5%, but it's not on the verge of extending human lifespan by 1000%.

It just isn't.

I'd bet my life on it.