The simplest way to test the claim that the Bible is the inerrant work of an omniscient creator is to compare its account with observable reality. When that is done, its creation stories reflect the limited biological knowledge of their human authors, not omniscience. Genesis 1:27 presents “male and female” as divinely ordained categories for humans, while the wider biblical narrative assumes the familiar arrangement of sexually reproducing males and females among animals; indeed in Genesis 6:19 the Bible unambiguously states all the animals on the Ark were male and female.
Real biology is much less tidy. Many animal species are simultaneous or sequential hermaphrodites; some lineages consist entirely of females and reproduce by parthenogenesis; and many organisms reproduce without sex at all. The biblical authors displayed only the knowledge expected of people living in an ancient pastoral society. An omniscient creator would have known better.
Nor are the mechanisms that determine sex fixed systems established in a single act of creation, with organisms thereafter reproducing only “after their kind”. They are themselves products of evolution. The new gecko study indicates that different lineages have switched repeatedly between genetic sex determination and environmental sex determination, in which conditions such as egg-incubation temperature influence the offspring’s sex. Even genetic sex determination has repeatedly been reinvented, with different ordinary chromosomes recruited independently to become X and Y or Z and W chromosomes. Instead of one immutable system imposed by a designer, geckos exhibit an evolutionary patchwork assembled and repeatedly modified over millions of years.
The familiar mammalian arrangement in which females are XX and males XY is sometimes treated as though it were the inevitable—and therefore designed—way of determining sex. But nature displays no such uniformity. Birds generally use a ZW system, in which females are ZW and males ZZ, while many reptiles dispense with specialised sex chromosomes altogether and allow environmental conditions, particularly incubation temperature, to influence whether an embryo develops as male or female.
Geckos exhibit all three arrangements, sometimes among relatively closely related species. This extraordinary diversity makes them a natural experiment in how sex-determination mechanisms originate, change and disappear during evolution.
In a new study published in *Science*, Yang Zhou and colleagues compared chromosome-level genomes from geckos representing all seven living families. Their reconstruction indicates that gecko lineages have switched between environmental and genetic sex determination at least 23 times. The researchers identified 22 distinct sex-chromosome systems and traced them to 17 different chromosomes in the ancestral gecko genome. Some of those ancestral chromosomes were recruited independently on several separate occasions.
In other words, the sex chromosomes of modern geckos were not inherited from a single, perfectly designed ancestral system. Evolution has repeatedly recruited ordinary chromosomes, turned them into sex chromosomes and then modified them as recombination was suppressed, genes were lost from the emerging Y or W chromosome, and compensatory changes in gene expression evolved.
These transformations were not entirely unconstrained. Chromosomes containing comparatively large numbers of genes preferentially expressed in testes were more likely to become Z chromosomes, whereas chromosomes with fewer such genes tended to become X chromosomes. When the researchers extended the comparison to 39 other vertebrates, a similar association appeared most strongly among internally fertilising amniotes.
This is precisely the sort of result that creationists may try to misrepresent. A finding that evolution is biased by existing genomic architecture might be described as showing that the outcome was “not purely random”, which can then be dishonestly translated into “directed” or “planned”. But those expressions do not mean the same thing.
Evolution always works with what already exists. Mutations and chromosomal rearrangements provide variation without anticipating what will be useful; inherited gene content makes some evolutionary routes more accessible than others; and natural selection preserves variants that reproduce successfully under the prevailing conditions. None of these processes knows what the eventual outcome will be, and none requires a rule-maker to prescribe it. A constrained process can produce statistically predictable patterns without possessing foresight or purpose.
The researchers also estimated that five independently arising sex-chromosome systems began differentiating within a broadly similar interval, centred on approximately 9.6 million years ago. This fell within a prolonged period of Miocene cooling, drying and habitat disruption. One possibility is that climatic instability made temperature-dependent sex determination less reliable, favouring genetic systems in some lineages. That remains a plausible hypothesis based on temporal association, however, rather than evidence that climatic change directly caused each transition.
What the study does demonstrate is more damaging to the creationist notion of immutable biological design. Gecko sex determination is an evolutionary patchwork: different chromosomes have repeatedly been recruited, rearranged, degraded and compensated for in different lineages. It is not one universal system imposed upon nature, but a collection of historically contingent solutions assembled by evolution from whatever genetic material happened to be available.
How is sex determined? Sex determination is the process that directs an organism towards one sexual developmental pathway rather than another. There is no universal mechanism. Evolution has produced numerous systems, and closely related species may use entirely different ones. The fact that sex determinism has evolved multiple time in multiple different ways is testament to its evolutionary importance, as we saw in a recent blog post.The paper in Science was accompanied by a Zhejiang University news release:
Sex need not remain fixed throughout life
System How it works Examples XX/XY chromosomes Females usually possess two X chromosomes, while males possess an X and a Y. In most mammals, the SRY gene on the Y chromosome initiates male development. Humans and most other mammals ZZ/ZW chromosomes Males possess two Z chromosomes, while females possess a Z and a W. The sex carrying two different chromosomes is therefore female—the reverse of the familiar mammalian arrangement. Birds, butterflies, moths, and some snakes, lizards and fish XX/X0 chromosomes Females possess two X chromosomes, while males have only one. The “0” denotes the absence of a second sex chromosome. Many grasshoppers, crickets and other insects Haplodiploidy Females usually develop from fertilised eggs and possess two sets of chromosomes. Males develop from unfertilised eggs and have only one set. Many bees, ants and wasps Temperature-dependent sex determination Temperatures experienced during a critical period of embryonic development influence which sexual pathway develops. The response differs between lineages: high temperatures produce females in many turtles, whereas American alligators produce males mainly at intermediate temperatures. Many turtles and crocodilians, the tuatara, and some lizards, including geckos Polygenic and mixed systems Sex is influenced by several genes rather than one dominant chromosome, sometimes in combination with temperature or other environmental conditions. Various fish, amphibians and reptiles
Some animals are sequential hermaphrodites and change sex as they mature or as their social circumstances change. Clownfish begin life as males; if the dominant female disappears, the largest male changes into a female. In many wrasses the process runs in the opposite direction, with a dominant female becoming male.
Other species, including many earthworms, slugs and snails, are simultaneous hermaphrodites, possessing both male and female reproductive systems. Some all-female whiptail-lizard lineages reproduce by parthenogenesis, producing offspring without fertilisation.
The letters X, Y, Z and W describe how particular chromosomes are inherited; they do not identify universal chromosomes shared by every species. An X chromosome in one lineage need not have the same evolutionary origin as an X chromosome in another. Ordinary chromosomes have repeatedly been recruited as sex chromosomes, demonstrating that sex-determination systems are evolutionary arrangements rather than fixed components of a universal biological design.
XY or ZW? Geckos Reveal Evolution's Hidden Playbook
Why do humans rely on XY chromosomes to determine sex, while birds use ZW — and many reptiles use neither, letting temperature decide instead? Sex determination is remarkably diverse across the animal kingdom. In genotypic sex determination (GSD), sex is encoded in the genome: XX individuals are female and XY males in humans and most mammals, while in the ZW system, found in birds and some reptiles, males are ZZ and females ZW. Many reptiles instead use environment-dependent sex determination (ESD), most commonly temperature-dependent sex determination (TSD). In leatherback turtles, higher incubation temperatures produce females, and in American alligators, males emerge only within intermediate temperatures. Remarkably, XY and ZW systems have evolved independently many times from different ancestral chromosomes, raising the question of what drived [sic] evolution down one path rather than the other.
Geckos: An Ideal Study Group
Geckos are an ideal model for studying sex chromosome evolution because they exhibit TSD, XY-GSD, and ZW-GSD, and their sex chromosomes span the full spectrum of differentiation, from young, nearly identical (homomorphic) pairs to ancient, highly distinct (heteromorphic) ones. Geckos have also switched between ESD and GSD at least 23 times, making them a natural laboratory for tracking sex chromosome origins in real time. To capture this diversity, Professor ZHANG Guojie's team at the Centre for Evolutionary & Organismal Biology, Zhejiang University School of Medicine, analyzed 22 gecko species—3 TSD, 6 XY-GSD, and 11 ZW-GSD—together with two outgroups, collectively covering about 70% of known independent sex chromosome origins in geckos.
A “Mosaic” of Sex Chromosomes
Geckos diverged from other lizards and snakes about 191 million years ago and their common ancestor carried 38 chromosomes (2n=38), none of which were dedicated sex chromosomes. By comparing genomes across species, the team traced the 22 distinct sex chromosome systems found in 20 gecko species with GSD back to just 17 ancestral chromosomes. Some ancestral chromosomes were recruited independently as sex chromosome multiple times; ancestral chromosome 16, for example, evolved into a Z/W pair in four separate gecko lineages. Yet, even sex chromosomes sharing the same ancestral origin often differ in exactly which regions became sex-differentiated (the sex-differentiated regions or SDRs), and the genes classically known to trigger sex-determination in other vertebrates are usually absent from these SDR, hinting that geckos may rely on entirely novel molecular switches to determine sex.
A Synchronized “Birth Cohort” of Sex Chromosomes
To find out when these sex chromosomes arose, the researchers estimated their ages from patterns of DNA sequence divergence. The results ranged widely, from under 6 million years in some species to more than 54 million years in others. Remarkably, five of 11 dated GSD species began differentiating their sex chromosomes within a narrow 7–12-million-year window, clustering around 9.6 million years ago. This timing coincides with the Middle Miocene Climatic Transition (MMCT), a period of dramatic global cooling, aridification, and habitat fragmentation. The convergence hints that climatic disruption may have repeatedly pushed unrelated gecko lineages away from temperature-sensitive TSD and toward more climate-proof, genetically fixed GSD, a shared evolutionary response the authors call a “sex chromosome birth pulse.”
XY or ZW: What Tips the Balance?
But what determines whether a newly forming sex chromosome becomes an X or a Z? The answer, it turns out, may already be written into the chromosome before it ever becomes a sex chromosome at all. The ancestral chromosome rich in genes preferentially expressed in the testis were more likely to evolved into Z chromosomes (paired with the female-limited W), whereas those with fewer such genes tend to become X chromosomes (paired with the male-limited Y). This pattern held up not just in geckos, but across 39 vertebrate species spanning mammals, birds, reptiles, amphibians, and fish — and it was strongest among amniotes with internal fertilization, such as mammals, birds, and reptiles, hinting that intense sexual selection on sperm may help steer this evolutionary decision.
Chromosome Differentiation, Gene Loss, and Dosage Balance
Once a sex chromosome pair stops recombining, the Y or W chromosome begins to lose genes, a slow process of genetic erosion. This creates an imbalance: an individual with only one functional copy of a gene (for example, a male with a degenerated Y) risks having too little of its product compared with an individual carrying two copies. To compensate, most gecko species with genetic sex determination partially restore this balance by boosting gene expression in the sex carrying the degenerated chromosome. Meanwhile, the genes that do survive on Y/W chromosomes are not necessarily link to reproductive or sex-specific traits, instead, they tend to be genes essential for basic cellular functions, indicating that a gene's chances of survival depend less on what it does for reproduction and more on how costly it would be to lose a working copy.
A Four-Stage Model of Sex Chromosome Evolution
Bringing these findings together, the team proposes a four-stage model for how a new sex-determination system is born and stabilized: first, climate disruption favors a shift from temperature-dependent to genetically fixed GSD; second, the ancestral gene content of the chromosome involved biases the outcome toward either an X or a Z system; third, recombination suppression drives the two sex chromosome to differentiate from one another; and fourth, gene loss on the Y/W is offset by dosage compensation, locking the new system in place. Taken together, the findings suggest that the dazzling diversity of sex-determination system across vertebrates is not the product of evolutionary chance, but the outcome of predictable interplay between climate, genomic predisposition, and long-term selective pressure.
Adapted and translated from the article written by RAN Hao
Translator: DING Chenwei
Publication:
The picture revealed by this study is not one of an immutable system established during a single act of creation. It is one of repeated evolutionary experimentation. Different gecko lineages have abandoned environmental sex determination, recruited different ordinary chromosomes as sex chromosomes, suppressed recombination between them, lost genes from emerging Y or W chromosomes and evolved compensatory changes in gene expression. There is no single gecko solution because evolution has repeatedly arrived at different workable arrangements.
The apparent concentration of several transitions around 9.6 million years ago raises the intriguing possibility that climatic instability helped make temperature-dependent sex determination less reliable. That remains a hypothesis rather than a demonstrated causal mechanism. If further evidence supports it, however, it will provide another example of populations responding to changed conditions without foresight: variants producing a more dependable balance of the sexes would leave more descendants, while less successful arrangements would disappear.
Nor does the discovery of predictable genomic biases provide any support for a supernatural rule-maker. Evolution is constrained by history because every new system must be constructed from genes and chromosomes inherited from ancestral populations. Consequently, some chromosomes are better candidates than others for recruitment as an X, Y, Z or W. Such bias makes certain outcomes more probable, but probability is not planning, constraint is not instruction, and natural selection is not an intelligent agent working towards a predetermined result.
Instead of the simple and fixed male-and-female scheme imagined by the authors of Genesis, biology presents a profusion of chromosomal, genetic and environmental mechanisms, even among closely related animals. Gecko sex chromosomes bear the marks of descent, modification, degeneration and repair accumulated over tens of millions of years. They are precisely what we should expect from an unguided evolutionary process working opportunistically with whatever material was available—and nothing like the products of a single, omniscient act of design.
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