The Incredible Unlikeliness of Being: Evolution and the Making of Us by Alice Roberts.
Heron Books, 2014 (ISBN: 978 1 84866 477 7)
A fascinating look about how a human develops from embryo to birth, and how this fits in with current evolutionary theory. If you get a chance to watch one of Alice Robert's documentaries (shown on the BBC TV channel) then do; she combines a passion for her subject with clear explanations.
Each of us begins as a single cell and ends up as a complex organism. Our bodies work, but have both flaws and successful parts. They also show our history as we still retain traces of very ancient ancestors in the way embryos develop. "No organism is 'designed' by evolution from scratch. It's all about tweaking and tinkering with what's already there." Alice Roberts.
Here are some fascinating things I learnt.
Our DNA contains a set of instructions for building an embryo; when particular genes are switched on, cells produce signalling proteins which tell other cells what to do next, and this is different for each species. New genes usually appear as duplicates of existing ones, and occur due to mistakes in copying DNA. Early organisms had one cluster of pattern generating Hox genes, while mammals have four clusters. Multiple genes may (a) be surplus to requirements and degenerate or disappear or (b) one fulfils its old function and the duplicate can be used for something else. Some genes have multiple functions and are switched on and off at different times doing a different job each time. Your body is not exclusively determined by your DNA, but is also a product of how you use your body.
The early embryo divides into three layers: the ectoderm will become skin and nerves, the mesoderm forms bones, muscles and blood vessels, and the endoderm lines the gut, lungs and bladder. The ectoderm initially forms the neural tube; if this fails to fuse together at the 'head' end, the embryo will have no brain and die at or soon after birth. If the neural tube fails to fuse properly lower down, the baby will be born with spina bifida, a condition which ranges in severity.
At birth a baby will have most of its neurones (brain cells) and far too many connections. Superfluous connections are pruned back as the brain develops, based on experiences; this continues lifelong. Mirror neurones enable us to imitate and learn from others. Most mammals have more than 1,000 genes for olfactory sensors (3% of the genome), but humans have less than 400 active smell genes (most primates have 300 to 400 active smell genes). Complex eyes and colour vision has evolved and disappeared in different species over time. It is easy to see where a human is looking due to the large amount of 'white' of the eye (the sclera) and contrast between this and the iris. We are born with attention to eye gaze.
Brains are very energy-hungry, consuming around 20% of our entire daily energy requirements while only making up 2% of our body mass. A 2011 study found that mammals rely either on being smart or having good energy stores (fat). Humans are unusual with big brains and only relatively fat. Our guts, while small for our body size, are standard for fruit/omnivore eaters, meaning we can be very flexible in our diets.
Our tongues are important for intelligible speech and different to other mammals, including primates. They are rounded and highly mobile, not long and flat like most other mammals. The human larynx is fairly standard for mammals, but the human voice-box is very low in the neck, giving a long vocal tract between vocal cords and lips. The larynx may have moved downward due to a flatter face and so as not to be squashed between tongue and spine, which has an S shape due to our habitual bipedalism.
The human skeleton is evidence of habitual bipedalism: our basin-shaped pelvis, relatively large hip and knee joints, inward angle at knee, shape of ankle joints, springy feet, short toes and big toe in line with other toes. We went from walking in the trees to walking on the ground. For larger bodied mammals it is better to hang from a branch or stand up on it and use arms to steady ourselves and reach for fruit. The human chest is barrel shaped, with the bottom of the ribcage curving inwards. The chest is wide, shoulder blades lower and further apart than other primates; this allows us swing our arms as a counter-balance while walking. We have a lumbar spinal curve and lower pelvis which is better for balancing upright and walking. Our long legs and short toes make us good at distance running.
Our arms and hands are exceptionally mobile. Our shoulder blade is only anchored by muscles allowing large range of movement. Our low and wide shoulders enable us to throw things (e.g. spears to catch animals). Forearm bones can move, so we can turn hand palm upwards. Muscles in thumbs specific to humans allow us to apply force with a tool gripped in the hand. We can also extend thumb to the side.
Most primate babies are highly developed at birth, while human babies are very helpless. Chimp brain at birth is 40% of adult one, human brain 30% of adult size. Can't move around or hang on to mother, and need to be carried for first year. This guarantees intensive contact between baby and parents/care givers.
Human gestation is more than a month longer than expected for a primate our size, but our babies still have a lot of growing to do. Miscarriages act as a natural screening programme; without this loss it is estimated that 12% of babies would be born with defects (the actual rate is just 2%). Labour starts when a woman's metabolic rate rises to 2.1 times the normal rate. At this point the energy demands of the foetus exceed the maternal capacity for supply. The female pelvis is only just wide enough for baby's head and broad shoulders, and three rotations of emerging baby are needed during birth. Fossil evidence shows female human pelvis has increased in width over time.
While other primates seek seclusion for birth, human mothers seek assistance. Even so, obstructed labour (rare in archaeological record) is still a problem in many human populations ( 3 to 6 per 100 births). It is higher in contemporary agricultural communities where childhood malnutrition affects the growth of girls (shorter women and smaller pelves). In countries with poorer obstetric care it is possible that natural selection may be acting now to favour (a) women with wide pelves despite malnutrition, or (b) smaller babies. In richer countries natural selection is side-stepped by caesarean births and midwife assistance.
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