HUMAN-animal hybrids could be used to grow lifesaving organs and replacement limbs in a landmark breakthrough for medical science – but some fear it may be mankind "playing God".
History has been made as it emerged David Bennett, 57, from Maryland in the US, underwent a first-of-its-kind surgery to have a genetically-modified pig heart implanted in his chest.
Bennett had terminal heart disease, and underwent the bizarre-sounding procedure as a last hope – and he is now doing well three days after the operation.
And in the past scientists have used the techniques to splice human and monkey embryos together, transplant fetal organs into rats, and even grown human ears on mice.
The idea of the strange experiments tests is for human-ready organs to be grow on mutant animals in lab conditions – helping to fill the transplant void left by organ shortages.
So-called "chimera" embryos and use of animal parts, known as xenotransplants, have been very controversial – with some feeling that scientists have crossed the line.
Medical Daily asked in 2016 if the research was a "Medical Miracle or Playing God?".
And in some countries the research is illegal, with experiments on human chimeras only being allowed in the US as recently as 2016.
Some accuse scientists of carrying out "Frankenstein" experiments, and the tests have not gone over well with many religious groups.
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For decades scientists have been researching how to carry out these operations safely and to save lives.
One of the most high profile cases before Bennett's was the case of Stephanie Fae Beauclair, know as Baby Fae.
Doctors transplanted the tot with a baboon heart – becoming the first infant to receive a "xenotransplantation".
The baby, who suffered from a rare heart disease, died 21 days later from what doctors believed was a mismatch in blood type with the baboon.
Bennett's pig heart transplant, performed by a team at the University of Maryland Medicine, is the latest step in a journey that started more than a century ago.
Back in 1905, a French surgeon called Princeteau inserterted slices of rabbit kidney into a child suffering from renal insufficiency.
"The immediate results were excellent. The volume of the urine increased; vomiting stopped," he wrote, as reported by the Institute for Laboratory Animal Research.
I want to live. I know it's a shot in the dark, but it's my last choice
However, he added, tragically "the child died of pulmonary congestion" 16 days later.
The following year, also in France, another surgeon called Jaboulay had two failed attempts at putting pig and goat kidneys in two different patients.
1923 saw something of a breakthrough, even if it came about by accident.
When he was unable to get hold of a human kidney for a patient who needed a transplant, Neuhof implanted an organ from a lamb.
Although the patient died nine days later, Neuhof insisted that the operation was "not necessarily a dangerous one, as had been supposed," and called for a follow-up.
In the 1960s, doctors at Tulane University in Louisiana tried to use chimpanzee kidneys on those who were suffering terminal diseases.
One of the recipients of the primate kidney lived for another nine months, going back to their job as a schoolteacher, and when they died, the transplanted organ showed no signs of being rejected by its host.
Back in 1997, Dr Jay Vacanti grew a human ear on a mouse's back using cartilage cells, sparking fury among animal rights and pro-life groups.
Five years later, he was already suggesting it might be possible to "grow" a human liver.
In April last year, "chimera" monkey embryos containing human cells were grown in a lab by a joint US-Chinese team.
And in 2015 scientists caused controversy after they implanted human fetal kidneys in rats.
Lead scientist Professor Juan Carlos Izpisua Belmonte of the Salk Institute, who helped make a similar human-pig hybrid in 2017, said the work could pave the way to solving severe shortages in transplantable organs, as well as help us understand more about early human development, disease progression and ageing.
And he insisted the study, published in the journal Cell, met current ethical and legal guidelines.
But not all scientists were convinced, with the University of East Anglia's Dr Anna Smajdor explaining: "Whether these embryos are human or not is open to question."
While Professor Julian Savulescu from the University of Oxford warned the research "opens Pandora's box to human-nonhuman chimeras".
Last September, the first-ever genetically engineered pig kidney donation to a brain-dead human was performed, with some degree of success.
This has led us up to the present day, and a breakthrough pig-human heart transplant.
Shortly after performing the operation, Dr Bartley Griffith said: "This was a breakthrough surgery and brings us one step closer to solving the organ shortage crisis.
"There are simply not enough donor human hearts available to meet the long list of potential recipients."
A day before receiving the experimental surgery, David said in a statement released by the university: "It was either die or do this transplant.
"I want to live. I know it's a shot in the dark, but it's my last choice."
In 2019, it was reported that family planning clinics in the US were supplying researchers with aborted foetus body parts that were fused with mice in drug-testing experiments.
Speaking at the time, Phelim McAleer, who produced the film Gosnell about a rogue abortionist, told Fox News: "Aborted babies' bodies are a very valuable commodity in today's America.
"Research institutions, elite universities, medical centres pay a lot of money for baby parts."
But Planned Parenthood spokesperson Eric Ferrero denied it had profited from the sale of body parts, and said women can give permission to donate their unborn foetus.
"At several of our health centres, we help patients who want to donate tissue for scientific research, and we do this just like every other high-quality health care provider does," he said.
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