\n By Laurence D. Hurst
\n Published On: October 17, 2025\n <\/p>\n
Humans aren\u2019t evolution\u2019s gold standard. Evolutionary geneticist Laurence D. Hurst explains how chromosomal errors, risky pregnancies, and a high mutation rate leave the human genome fragile.<\/p>\n
To Shakespeare\u2019s Hamlet we humans are \u201cthe paragon of animals\u201d. But recent advances in genetics are suggesting that humans are far from being evolution\u2019s greatest achievement.<\/p>\n
For example, humans have an exceptionally high proportion of fertilized eggs that have the wrong number of chromosomes and one of the highest rates of harmful genetic mutation.<\/p>\n
In my new book\u00a0The Evolution of Imperfection<\/a>\u00a0I suggest that two features of our biology explain why our genetics are in such a poor state. First, we evolved a lot of our human features when our populations were small and second, we feed our young across a placenta.<\/p>\n Our reproduction is\u00a0notoriously risky<\/a>\u00a0for both mother and embryo. For every child born\u00a0another two fertilized eggs<\/a>\u00a0never made it.<\/p>\n Most human early embryos<\/a>\u00a0have chromosomal problems. For older mothers, these embryos tend to have\u00a0too many or too few<\/a>\u00a0chromosomes due to problems in the process of making eggs with just one copy of each chromosome. Most chromosomally abnormal embryos don\u2019t make it to week six so are never a\u00a0recognised pregnancy<\/a>.<\/p>\n About 15 percent of recognized pregnancies spontaneously miscarry<\/a>, usually before week 12, rising to 65 percent in women over 40. About half<\/a>\u00a0of miscarriages are because of chromosomal issues.<\/p>\n Other mammals have similar chromosome-number problems but with an error rate of about\u00a01 percent per chromosome<\/a>. Cows should have 30 chromosomes in sperm or egg but about 30 percent of their fertilized eggs have odd chromosome numbers<\/a>.<\/p>\n Humans with 23 chromosomes should have about 23 percent of fertilized eggs with the wrong number of chromosomes but our rate is higher in part<\/a>\u00a0because we presently reproduce late and chromosomal errors escalate with maternal age.<\/p>\n Survive that, then gestational diabetes and high blood pressures issues await, most notably pre-eclampsia, potentially lethal to\u00a0mother and child<\/a>, affecting about 5 percent of pregnancies. It is unique to humans.<\/p>\n Historically, up until about 1800, childbirth was remarkably dangerous with about 1 percent maternal mortality<\/a>\u00a0risk, largely owing to pre-eclampsia, bleeding and infection. In\u00a0Japanese macaques<\/a>\u00a0by contrast, despite offspring also having a large head, maternal mortality isn\u2019t seen. Advances in maternal care have seen current UK maternal mortality rates plummet to\u00a00.01 percent<\/a>.<\/p>\n Many of these problems are contingent on the placenta. Compare us to a kiwi bird that loads its large egg with resources and sits on it, even if it is dead: time and energy wasted. In mammals, if the embryo is not viable, the mother may not even know she had conceived.<\/p>\n The high rate of chromosomal issues in our early embryos is a\u00a0mammalian trait<\/a>\u00a0connected to the fact that early termination of a pregnancy lessens the costs, meaning less time wasted holding onto a dead embryo and not giving up the resources that are needed for a viable embryo to grow into a baby.<\/p>\n But reduced costs are\u00a0not enough<\/a>\u00a0to explain why chromosomal problems are so common in mammals.<\/p>\n During the process of making a fertilizable egg with one copy of each chromosome, a sister cell is produced, called the polar body. It\u2019s there to discard half of the chromosomes. It can \u201cpay\u201d in evolutionary terms<\/a> for a chromosome to not go to the polar body when it should instead stay behind in the soon to be fertilized egg.<\/p>\n It forces redirection of resources to viable offspring. This can\u00a0explain why<\/a>\u00a0chromosomal errors are mostly maternal and why, given their lack of ability to redirect saved energy, other vertebrates don\u2019t seem to have\u00a0embryonic chromosome problems<\/a>.<\/p>\n Our problems with\u00a0gestational diabetes<\/a>\u00a0are a consequence of foetuses releasing chemicals from the placenta into the mother\u2019s blood to keep\u00a0glucose available<\/a>. The problems with pre-eclampsia are associated with malfunctioning placentas, in part owing to\u00a0maternal immune rejection<\/a>\u00a0of the foetus.<\/p>\n Regular unprotected sex can protect women\u00a0against pre-eclampsia<\/a>\u00a0by helping the mother become used to paternal proteins. The fact that pre-eclampsia is\u00a0human-specific<\/a>\u00a0may be related to our exceptionally invasive placenta that burrows deep into the uterine lining, possibly required to build our unusually large brains.<\/p>\n Our other peculiarities are predicted by the most influential evolutionary theory of the last 50 years, the\u00a0nearly-neutral theory<\/a>. It states that natural selection is less efficient when a species has few individuals.<\/p>\n A slightly harmful mutation can be removed from a population if that population is large but can increase in frequency, by chance, if the population is small. Most human-specific features evolved when our population size was around\u00a010,000<\/a>\u00a0in Africa prior to its recent (last 20,000 years) expansion. Minuscule compared to, for example, bacterial populations.<\/p>\n This\u00a0explains why<\/a>\u00a0we have such a\u00a0bloated genome<\/a>. The main job of DNA is to give instructions to our cells about how to make the proteins vital for life.<\/p>\n That is done by just 1 percent of our DNA but by 85 percent of that of our gut-dwelling bacteria Escherichia coli. Some of our DNA is required for other reasons, such as controlling which genes get activated and when. Yet\u00a0only about 10 percent<\/a>\u00a0of our DNA shows any signs of being useful.<\/p>\n If you have a small population size, you also have more problems stopping genetical errors like mutations. Although DNA mutations can be beneficial, they are more commonly a curse. They are the basis of genetic diseases, be they complex (such as Crohn\u2019s disease and predispositions to cancer), or owing to single gene effects (like cystic fibrosis and Huntington\u2019s disease).<\/p>\n We have one of the\u00a0highest mutation rates<\/a>\u00a0of all species. Other species with massive populations have mutation rates\u00a0over three orders of magnitude lower<\/a>, another prediction of the\u00a0nearly-neutral theory<\/a>.<\/p>\n A consequence of our high mutation rate is that around\u00a05 percent of us suffer<\/a>\u00a0a \u201crare\u201d genetic disease.<\/p>\n Modern medicine may help cure our many ailments, but if we can\u2019t do anything about our mutation rate, we will still get ill.<\/p>\n This article by Laurence D. Hurst<\/a>, professor of Evolutionary Genetics at the University of Bath,\u00a0is republished from\u00a0The Conversation<\/a>\u00a0under a Creative Commons license.<\/p>\n","protected":false},"excerpt":{"rendered":"By Laurence D. Hurst Published On: October 17, 2025 Humans aren\u2019t evolution\u2019s gold standard. 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