Sunday, July 17, 2016

Suzana Herculano-Houzel's The Human Advantage is a good book with some important insights hidden behind a fairly dry and dense presentation on "how I made these important discoveries." Herculano-Houzel (by her own account) pioneered a technique for determining the number of neurons in brain tissue, and managed (through a fair number of mildly interesting adventures) to bring together samples of many different primate, rodent, and other mammalian brains in order to work out the scaling laws that govern how brains and neuron counts grow with body mass in different tissues across different lineages. She shows a lot of graphs and charts to demonstrate that (with two exceptions) for most mammals, neuron counts scale up with an exponent of .5 with body mass, but in primates, the scaling factor is .8. If neurons have to be added in order to increase intelligence, this means that primates have a huge advantage. In order to get smarter, brains and neuron count have to increase. Larger bodies are necessary in order to sustain a larger brain, and if neuron count or neuron density is the limiting factor in intelligence, then you want to be able to pack more neurons into a denser brain in order not to require an enormous body.

Once Herculano-Houzel has established the basic scaling laws, she delves into the economics of maintaining a sufficiently neuron rich brain. The comparative scaling laws mean that as you look at bigger and bigger species of mammals, the neuron counts increase with the square root of the body mass. As primate species get larger, their neuron count increases much more quickly, which means that for a given body size, a primate is capable of supporting a larger brain than would a mammal. The cross-over point where the two lineages have comparable neuron sizes are with body sizes in the 10 gram range. At any larger size, if you compare a primate with another similar sized mammal, the primate is probably smarter.

There is a separate literature showing the energy requirements for many particular species. Herculano-Houzel used her new numbers on neurons for rodent and primate species to show that there is a direct correlation between the number of neurons and the amount of glucose consumed per minute by the brain. For humans (and others in the Homo lineage, being able to take advantage of the primate scaling laws gives a big boost, but you still have to find a way to ingest sufficient calories to afford the bigger brain.

Humans have two main advantages on this score compared to other primates. Walking on two legs is much more energy efficient than knuckle-walking like other (primarily arboreal) primates or on four legs. This increases the range over which foragers could range, and also freed up hands for gathering and carrying. It's not clear what originally drove bipedalism in the homo line, but it occurs at the same branch point that leads to the massive growth in cranial capacity.

The other big human advantage is cooking. I've seen discussions before that cooking increases the efficiency of digestion, and led to our shorter digestive tract, which allowed us to switch energy resources from digestion to our brains, but Herculano-Houzel points out that even before control of fire, other kinds of preparation (chopping and mashing for example) reduce the energy required for digestion. The anthropological evidence for food preparation goes back much further than the evidence of cooking, and significantly after bipedalism. The earliest evidence of eating meat is swiftly followed by anatomic adaptations to a more efficient diet, which is quickly followed by better tools, and the then bigger brains. Part of the evolutionary adaptation for bigger brains included smaller jaws.

Evidence of tool use and manufacture date back to 3.3 MYA (Million Years Ago). This date was recently pushed back from 2.6 MYA. These tools were simple flint knives. Archaeologists wouldn't count rocks that were used for pounding, since they are impossible to distinguish from unworked rocks. The flint knives would have been useful for cutting up meat, which would make it more digestible, and is necessary in order to survive with smaller jaws. Presumably, eating primitively processed foods had to become habitual before later evolutionary steps that relied on it would have survived in the population. The archaeological evidence gives the following timeline:

  1. 4.4 MYA: bipedalism appears
  2. 3.3 MYA: earliest tool use
  3. 2.5 MYA: eating meat
  4. 2.4 MYA: beginning of the reduction in size of the jaw
  5. 1.9 MYA: smaller gut is clearly present
  6. 1.7 MYA to 300 KYA: The Acheulean hand axe
  7. 1.5 MYA-100 KYA: start of the increase in cranial capacity
  8. 1 MYA: Clear indications of cooking

Another tantalizing clue is that the taste for cooked food may pre-date adoption of the habit. Herculano-Houzel refers to two studies that show that chimpanzees have a very strong preference for the taste of cooked food over raw. I don't know whether this has been investigated in other lineages, but if so, (even if it's just the body innately being able to detect foods that are provide big efficiency gains) it provides a boost for any lineage that can figure out how to reliably prepare foods--once you start, it would be an easy habit to keep, providing that the right food sources and tools are accessible.

Earlier, I mentioned that there are two exceptions to the laws regulating the number of neurons in primates and in all other mammals. The first is gorillas, which have brains and neuron counts much closer to those of other mammals rather than those expected of a primate. This fact about gorillas has been throwing off the results of previous researchers, who could only measure brain capacity. They concluded that the rules for primates would be the same as for other mammals, and argued that it was humans that were outliers. Once you plot the detailed data from small and medium primates and compare to mammals, it's easy to see a different trend line applies, and that humans fit on the primate line and gorillas do not. The other exception is elephants. (Herculano-Houzel has an entertaining section about her adventures getting elephant brains to analyze.) Elephants have brains whose size follows the standard scaling rule for mammals. They're huge, and they have huge brains. But their neurons are distributed very differently from all other species. 98% of the neurons are in the cerebellum, while the normal number doesn't get much above 80%. So elephants have big brains and a lot of neurons, but this explains why they're not even smarter than us, presuming neurons in the cerebral cortex are the thing that matters most.

Anyway, the later clues about cooking and bipedalism only added to my reaction that this work may provide an improved answer to the Fermi paradox. Herculano-Houzel doesn't appear to have data about the brains of animals beyond mammals, but if all the mammals outside of primates share a common scaling factor, then that's an indication that it's hard to evolve intelligence given the standard energy budget. It takes a special trick (which didn't have an immediate obviously benefit in the small primates in which it evolved) which was only discovered in one previously obscure branch of the mammal family tree to enable the efficient scaling that allows bodies to grow large enough to support brains supporting enough neurons to enable tool use. This enables (with other accidents like bipedalism and prepared food appearing in the same lineage) the feedback cycle that led to our massive growth in intelligence.

I've never been very worried by the argument that says the Fermi paradox indicates that there's a Great Filter, and if we can't figure out what the hard step was in our past, we should expect to encounter a hurdle in our future that has stopped other species from getting to space. The Human Advantage makes me even more sanguine. It's hard to evolve an intelligent species. There are a lot of happy accidents in our past, and the likely number of extra-terrestrial species in our light cone may be smaller than we thought. It would be nice to see more data showing the scaling laws that apply outside the primates (and in the cetaceans, which she didn't give much data about). I'll be surprised if any of them show divergent scaling progression compared to baseline mammals.

Thursday, July 14, 2016

Jo Walton's The Just City investigates the nature of justice, while telling an enchanting story involving gods, mortals and robots trying to actually build the Just City hypothesized by Plato. The goddess Pallas Athene brings together 10,000 children and a few hundred adults from many different times and places to found a new city (on the island of Atlantis) according to Plato's prescriptions in The Republic. Plato's main goal in The Republic was to explain the nature of Justice (the words 'Just' and 'Justice' occur more than 150 times on the wikipedia page), so Walton has plenty of room to explore the idea from several different directions.

In order to get infrastructure in place without burdening the new inhabitants (who are supposed to be coddled and trained so they can understand Justice) the goddess brings in robots to build housing and meeting rooms. The robots are kept around to take care of maintenance and other tasks that Plato didn't describe the city's inhabitants as handling. When the historical Socrates (as envisioned by Walton) joins the city, he turns out to be a very inquisitive man. Since the robots display some autonomy, Socrates wonders whether they have individual personalities and whether they're thinking and aware. This leads to even more opportunities for questions about Justice.

Pallas Athene wants to populate the city with willing participants, so the children (exactly half girls and half boys) all are from disadvantaged circumstances. The adults are all people who prayed for a chance to live in Plato's Republic, so (considering how often the book is actually read in the original greek, and in what historical periods) most are men from antiquity, and the women are nearly all from more modern times. This leads to some interesting political factions, and changing of practices as time goes on and the oldest denizens die off first.

Not all of the children are happy to be there, even though all of them agree that their previous lives (most were slaves) were worse. Even so, not having been given a choice rankles with a few, and their reactions are also interesting.

Many of Plato's ideas are reasonably modern, but others are very outdated, like assigning citizens to societal roles according to their metal. The adults of The Just City spend a lot of effort training and testing the children in order to place them appropriately. Many of the adults are uncomfortable with this duty, but they carry it out, and even put their thumbs on the scales as necessary in order to make the numbers come out right according to Plato's very Greek ideas about numerical harmony. When some of the children figure out that test rankings are being adjusted in order to fit pre-defined notions of how many should be in each category, they challenge the adults, and as with everything else that goes on in the city, philosophical discussion and socratic dialogue ensues.

Since there's limited space on the island, procreation must be limited and sexual activity controlled. The children (and adults) find creative ways around the restrictions, but this means discussion of sexual mores and prohibitions are necessary. We hear about everything from rape and unwanted intimacy to Plato's ignorance of issues of women's hygiene. For the most part the adults attempt to do everything according to Plato's prescriptions, but there are several clear gaps in Plato's planning which leads to the need for endless committee meetings (most of which we, mercifully, hear about afterward, rather than having to endure.)

Walton does a wonderful job of presenting these philosophical questions of freedom, choice, and justice through the character's activity and interactions. In the end, we get to know these people who are all striving to be their best, and to create an environment in which justice is available to all, even though humans always have incompatible desires. We even get some satisfying answers to new and old questions and some unresolved issues to ponder on our own.

Tuesday, July 05, 2016

To Explain The World: by Steven Weinberg

Steven Weinberg's To Explain The World provides a very good introduction to the age of scientific discovery. Weinberg's goal is to explain how the early scientists incrementally developed the scientific method as they were learning, developing new theory, and testing their ideas, but I thought the presentation also did an amazing job of explaining the context of the discoveries in a way that made each contribution much more understandable. I've read many histories of science, so few of the foreground facts (who discovered what and when) were new to me, but being reminded about what else was going on at the time, and which people had been talking to (or arguing with) one another gave more context and made it easier to judge the relevance and difficulty of their accomplishments.

Weinberg starts with the ancient greeks, and explains how they were interested in the nature of reality, and our place in the cosmos, but had no concept of comparing their proposed explanations to the world or in any way testing their ideas. In some ways, some of those whose names live on were proposing better solutions than any that would appear for thousands of years, but without demonstrations of their truth or applicability, they wouldn't be influential until rediscovered in later circumstances, where better scientific methods would allow them to take part in a system of understanding.

In the hellenistic period in Egypt after the death of Alexander, individuals were able to figure out that falling objects accelerate, that air is a real substance that can displace water, and to invent effective pumps and accurate water clocks. When they made useful artifacts, their ideas had consequences and were remembered. When they theorized ex nihilo, they were more evanescent and didn't effect many later thinkers. Around this time, Archimedes systematized approaches to the simple machines, and recorded drawings of many useful tools built from screws, ramps, levers, and pulleys.

Starting from their certainty that heavenly bodies must move in circles, Plato asked some of his students what combinations of circular motions could produce the wandering motions of the seven known planets against the starry background that the Greeks could see with their unaided eyes. The pattern of inventing and compounding epicycles would continue until the time of Kepler.

Weinberg's final word on Leibniz and Newton is illustrative of how he analyzes the interactions between scientists, and looks for their influences on one another and on progress generally.

The judgement of contemporary scholars is that Leibniz and Newton had discovered the calculus independently. Newton accomplished this a decade earlier than Leibniz, but Leibniz deserves great credit for publishing his work. In contrast, after his original effort in 1671 to find a publisher for his treatise on calculus, Newton allowed this work to remain hidden until he was forced into the open by the controversy with Leibniz. The decision to go public is generally a critical element in the process of scientific discovery. It represents a judgement by the author that the work is correct and ready to be used by other scientists. For this reason, the credit for a scientific discovery today usually goes to the first to publish. But though Leibniz was the first to publish on calculus, as we shall see it was Newton rather than Leibniz who applied calculus to problems in science. Though, like Descartes, Leibniz was a great mathematics whose philosophical work is much admired, he made no important contributions to natural science.

Weinberg gives credit to the Greeks for discovering and passing on the idea that nature follows consistent rules, and that we can use mathematics to build models which will help us explain and understand them. It wasn't until the 17th century however, that scientists realized that this can be applied pervasively, and actively looked for opportunities to explore new phenomena and describe them mathematically. The final two thirds of the book is a discussion of the conversation that arose among scientists as they investigated, shared observations, and looked for ways to apply fewer and broader explanations to more and more fields.