Patrick Tschopp/Harvard Medical School A python embryo with budding genitals and vestigial limb-buds. 
 
 When the first animals scrambled out of the water to live on land, they needed limbs and lungs. And something else: a new way to reproduce. 
 
 On terra firma, prospective parents no longer had the luxury of an aquatic environment where eggs could be laid and sperm released over them. 
 
 Advertisement 
 
 Evolution elegantly solved the problem, but the solution did not always take the same form. Crocodiles, birds, and mammals have one penis. Lizards and snakes have two. 
 
 The sex organs of animals are some of evolution’s most diverse handiwork. Yet until now, scientists have not understood exactly how they evolved and took shape at the earliest stages of development. 
 
 In a study published Wednesday in the journal Nature, researchers at Harvard Medical School have begun to unravel how genitals form. Their tools range from standard biomedical techniques, such as examining which genes are flipped on during embryonic development, to Frankenstein-like surgeries in which they performed surgical transplants to see whether they could trigger genitals to bud in unusual, out-of-the-way spots. 
 
 The researchers found that genitals develop from the same cells that give rise to hind legs in lizards or that form the remnants of limb buds in snakes. In mammals and birds, a different set of cells closer to the tail area give rise to genitalia. Despite the different source materials, they found the process of genitalia formation is deeply similar among those creatures, triggered by the same basic genetic programs and signals. That process is, in turn, similar to the way limbs form, suggesting that genitals and limbs share evolutionary origins. 
 
 “It’s clearly really basic research, mostly about evolution and development. But what is interesting is it has been known in the medical field that babies that are born with malformations in their limbs often also have malformations in their genitalia,” said Patrick Tschopp, a postdoctoral researcher at Harvard who led the research. “We knew there was some sort of genetic link between the two, and this could provide some information about where these genetic links are.” 
 
 Tschopp began working on the question in Clifford Tabin’s genetics laboratory at Harvard, where researchers study how animals evolved different forms. A colleague studying limb development had been trying to figure out whether snakes lacked the basic developmental mechanism that gave rise to limbs. 
 
 Tschopp got interested when researchers noticed the cellular program that gave rise to snakes’ unusual, two-pronged genitalia looked very similar to limb development. 
 
 The researchers began to study which genes were responsible for the development of genitals and to pinpoint the cells in the embryo from which they originated. 
 
 Once it was clear that two different sets of cells were the origins of genitals, they tried to figure out why. They noted that the cloaca, a precursor to the gut, varied in its position in reptiles and mammals, and theorized that the origin of the genitals might be due to signals sent by the cloaca to surrounding cells. 
 
 When they transplanted a cloaca onto the cells that typically give rise to hind limbs in a chicken embryo, they were able to cause genital-like buds to form there. 
 
 That shows genitalia were not passed from a common ancestor to reptiles, mammals, and birds, but they are biologically related in a deep way, formed in response to signals from the same ancient source: the cloaca. 
 
 Dr. Martin Cohn, a professor of developmental biology at the University of Florida, simultaneously published a paper in the journal Scientific Reports reporting that genitals start as two buds that eventually migrate toward the center line of the body. In some animals, such as the chick embryos Cohn studied most recently, they join up. In others they don’t, creating the bizarre double “hemipenes” of reptiles. 
 
 Despite the fact that scientists are interested in studying the origins of genitalia for purely scientific reasons, both scientists said it makes for better fodder when people ask about their work. 
 
 “In terms of the questions, I’m most interested in how changes in gene regulation can shape evolution,” Tschopp said. “But working on external genitalia makes for better cocktail conversations than before, when I [worked] on digits.” ||||| Image copyright Spl Image caption Lizard males have paired genitals called hemipenes - now research shows their genetic origin comes from limbs 
 
 In order for vertebrates to evolve from the sea to the land, some drastic evolutionary changes were needed. 
 
 Their ancient sea-dwelling ancestors had no need for external sex organs whereas their land relatives did. 
 
 Now a new study offers insights into the genetic changes that allowed land-dwelling animals to develop sex organs. 
 
 The Nature research suggests the key to the origin of genitalia lies in the limbs, at least in snakes and lizards. 
 
 For their genitals - called hemipenes - to develop, a signalling centre instructs the relevant genes to switch on. 
 
 Initially the researchers wanted to understand why snakes do not develop limbs but then soon discovered that the earliest stages of genital development closely resembled limb formation. 
 
 By misplacing a molecular signal you can misguide these cells in their developmental trajectory Dr Patrick Tschopp, Harvard University 
 
 They found that when a given nudge, embryonic limb cells of lizards and snakes could then be turned into genitals. In mice, tail bud cells could be manipulated in a similar way. 
 
 'Genital fate' 
 
 The team did this by moving the position of "a signalling source" called the cloaca - a transient embryonic structure which gives off signalling molecules informing genes to switch on or off. 
 
 "It demonstrates that there is a flexibility with what kind of cells can get recruited during development to form genitalia," explained lead author of the research, Dr Patrick Tschopp from the Harvard Medical School in Cambridge, US. 
 
 Image copyright Patrick Tschopp Image caption The hemipenes (genitalia) bud can be seen at the tail end (bottom right) of this house snake embryo 
 
 "What we were able to show is that if you ectopically transplant this cloaca into either limb or tail bud cells, these cells respond in a way that reflect their development being redirected to a genital fate," he added. 
 
 "In other words, by misplacing a molecular signal you can misguide these cells in their developmental trajectory," Dr Tschopp told BBC News. 
 
 In order to change these cells' fates, they traced the cell populations that form genital organs during development. 
 
 Recruiting cells 
 
 Then they analysed the genetic components of the embryonic cells to identify which genes were turned on and off by extracting and sequencing RNA molecules, the messengers from each gene. 
 
 The study also found that in mice, the sex organs had genetic origins in the tail bud. The researchers say that this occurred due to a differing position of the cloaca which changed the cells it could "recruit" to form genitalia. 
 
 Dr Tschopp explained that genital evolution was another adaptive measure vital for living on land, in a similar way to how limbs have an evolutionary origin in fish fins. 
 
 Commenting on the paper, developmental biologist Dr Liang Ma of Washington University in St Louis, US, said the work was fantastic, and that it was highly important for the fields of both genitalia and limb research. 
 
 Image copyright Spl 
 
 "This paper dealt with the longstanding unresolved issue of the origin of genitalia. It turns out that the mouse is the odd one out, it was not similar to the snakes or the chicken. 
 
 "This paper provides a new twist to a previous hypothesis that genitals and limbs share a deep homology [shared ancestry], it provides formal evidence of how this co-evolution between the two structures can happen in an organism." 
 
 In a related study published in journal Scientific Reports, a team looked at the embryonic origins of chicken genitals. They used fluorescent dyes to label small populations of cells in the chicken embryo and follow them to development. 
 
 Evolutionary origins 
 
 They found that two paired populations of cells are brought together into one structure to form the chicken's sex organs. 
 
 Co-author of this paper, Prof Martin Cohn of the Howard Hughes Medical Institute in Maryland, US, said that because the evolution of external genitals occurred more rapidly than any other organ, there was a tremendous amount of diversity of anatomical form. "Because of the rapid evolution, people don't really understand how it's controlled. 
 
 "Taken together, I think the papers highlight a very deep evolutionary conservation of the earliest steps in genital development. It appears that the way all amniotes, including reptiles, birds and mammals, build their genitalia is very similar. 
 
 "What these molecular and genomic tools allow us to do is compare the degree of relatedness, not only of the structures and the organisms, but of the genetic pathways that build those structures," Prof Cohn added. 
 
 Follow Melissa on twitter ||||| A house snake embryo at six days after oviposition (after eggs are deposited or laid). The right genitalia bud can be seen near the tail end of the embryo. 
 
 Animal penises vary widely -- snakes and lizards have two while birds and humans have one -- yet a new study finds that they function the same way and express similar genes. 
 
 The research, reported in the journal Nature, helps explain the genesis of external genitalia. It turns out that reptile and mammal genitalia have at least some things in common. 
 
 The researchers found that the development of limbs and external genitalia are linked. In snakes and lizards, the external genitalia are made from the same tissue from which the reptile’s hind limbs (or remnants, as with snakes) originate. 
 
 In mammals, on the other hand, they are made from what’s known as “tail-bud” tissue. This is a knob of embryonic tissue that contributes to the formation of the posterior part of our and other mammals’ bodies. 
 
 "While mammal and reptile genitalia are not homologous in that they are derived from different tissue, they do share a 'deep homology' in that they are derived from the same genetic program and induced by the same ancestral set of molecular signals," lead author Clifford Tabin said in a press release. 
 
 Tabin, a professor at Harvard Medical School's Department of Genetics, and his team examined how genitalia first develop in the embryos of certain animals, including snakes and chickens. 
 
 Before the genitalia emerge, the embryonic cloaca (tissue that eventually develops into the urinary and gut tracts) issues molecular signals that tell neighboring cells and tissues to form into external genitalia. The researchers discovered that the cloaca's location determines which tissues receive the signal first. 
 
 In reptiles, the cloaca is located closer to the hind limb tissues. In mammals, the cloaca is closer to the tail-bud, helping to explain why those regions later support the development of external genitals. 
 
 Another cool aspect of the research: It shows how much snakes have evolved over the years. Snakes used to have legs -- they likely evolved from a lizard that either burrowed on land or swam in the ocean. The legs must have disturbed the movement, rendering them useless over time. 
 
 Although snake legs are long gone, snakes' external genitalia emerge near where their hind legs used to be.
– Most guys probably don't sit around the locker room pondering the finer points of how the male penis evolved, so leave this one to Harvard researchers: The mystery is solved thanks to … lizard limbs. Yup, Harvard scientists investigating the origin of external genitalia have found a link relating to the development of limbs in a variety of animals. Scientists initially hoped to discover why snakes didn't grow limbs, the Boston Globe reports. But they instead found that embryonic cells—which produce hind limbs in lizards, limb buds in snakes, and "tail-bud" tissue in mammals—form genitalia with a signal from something called a cloaca. As Discovery reports, cloaca is "tissue that eventually develops into the urinary and gut tracts." When they transplanted a cloaca onto hind limb cells in a chicken embryo, eureka—genital-like buds formed. "It demonstrates that there is a flexibility with what kind of cells can get recruited during development to form genitalia," the lead author tells the BBC of his research in Nature. He says the link may help explain why "babies that are born with malformations in their limbs often also have malformations in their genitalia." While snakes and lizards have two penises, "it appears that the way all amniotes, including reptiles, birds, and mammals, build their genitalia is very similar," a researcher says. The Globe adds that external genitalia developed when animals moved from sea to land, rendering water-borne fertilization obsolete. (Another study finds sex is 385 million years old.)