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Muscles discovered in unborn babies’ hands and feet disappeared from adults 250million years ago

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Human foetuses grow ‘reptile muscles’ in their hands and feet which are a legacy of our ancestors from 250million years ago

  • Scans reveal in unprecedented detail the now-redundant muscles in foetuses
  • The muscles were phased out during transition from reptiles to mammals
  • Almost all of them are now absorbed into other muscles before birth 
  • They are rarely found in adults but may be left behind by genetic disorders 

Muscles discovered in the hands and feet of unborn babies are part of the human body which was phased out by evolution 250million years ago.

Scientists have discovered small muscles in the limbs of foetuses less than two months old which don’t exist in adults.

The muscles are found in other mammals with limbs but not in people, and were believed to have been phased out when we evolved from reptiles into mammals.

High-tech imaging machines, however, can now spot them in unprecedented detail in developing foetuses.

The researchers said they could be seen ‘strikingly late’ in the development stages for something which had apparently been made redundant.

Scans of unborn babies revealed the existence of muscles called dorsometacarpales (Pictured: the left hand of a 10-week-old human embryo), which researchers said disappeared in adults when human evolution progressed from reptiles to mammals

Scans of unborn babies revealed the existence of muscles called dorsometacarpales (Pictured: the left hand of a 10-week-old human embryo), which researchers said disappeared in adults when human evolution progressed from reptiles to mammals

Scientists from Howard University in Washington DC revealed the discovery of the muscles, some of which were named dorsometacarpales.

These were found between the bones of the hand, which are called metacarpals.

By the seventh week of a pregnancy, the foetuses in the study had developed about 30 of these hand and foot muscles.

By 13 weeks, however, about 10 of them had disappeared already, and the rest of them would disappear over time.

They are only found in adults in rare cases of as the result of a genetic disorder, the scientists said.

Dr Rui Diogo, who led the study, said: ‘Some of the muscles are found on rare occasions in adults, either as anatomical variations without any noticeable effect for the healthy individual, or as the result of congenital malformations.

‘This reinforces the idea that both muscle variations and pathologies can be related to delayed or arrested embryonic development, in this case perhaps a delay or decrease of muscle [cell death] and helps to explain why these muscles are occasionally found in adult people.

‘It provides a fascinating, powerful example of evolution at play.’

WHEN DID HUMAN ANCESTORS FIRST EMERGE?

The timeline of human evolution can be traced back millions of years. Experts estimate that the family tree goes as such:

55 million years ago – First primitive primates evolve

15 million years ago – Hominidae (great apes) evolve from the ancestors of the gibbon

7 million years ago – First gorillas evolve. Later, chimp and human lineages diverge

A recreation of a Neanderthal man is pictured 

A recreation of a Neanderthal man is pictured 

5.5 million years ago – Ardipithecus, early ‘proto-human’ shares traits with chimps and gorillas

4 million years ago – Ape like early humans, the Australopithecines appeared. They had brains no larger than a chimpanzee’s but other more human like features 

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3.9-2.9 million years ago – Australoipithecus afarensis lived in Africa.  

2.7 million years ago – Paranthropus, lived in woods and had massive jaws for chewing  

2.6 million years ago – Hand axes become the first major technological innovation 

2.3 million years ago – Homo habilis first thought to have appeared in Africa

1.85 million years ago – First ‘modern’ hand emerges 

1.8 million years ago – Homo ergaster begins to appear in fossil record 

800,000 years ago – Early humans control fire and create hearths. Brain size increases rapidly

400,000 years ago – Neanderthals first begin to appear and spread across Europe and Asia

300,000 to 200,000 years ago – Homo sapiens – modern humans – appear in Africa

50,000 to 40,000 years ago – Modern humans reach Europe 

Scientists think the muscles could date back to before humans' evolutionary ancestors turned from reptiles into mammals. Illustrated: Pelycosaurs, which were classed as 'mammal-like reptiles' and believed to be predecessors to modern-day mammals

Scientists think the muscles could date back to before humans’ evolutionary ancestors turned from reptiles into mammals. Illustrated: Pelycosaurs, which were classed as ‘mammal-like reptiles’ and believed to be predecessors to modern-day mammals 

The muscles are described as atavistic, meaning they were found in ancient evolutionary ancestors.

Other examples of atavistic body parts are found throughout nature – ostriches and other flightless birds still have wings; dolphins, porpoises and whales begin to develop legs in the womb but then abandon them.

Even in humans, the tailbone or coccyx is a remnant of when we used to grow tails.

It has in the past been difficult to find these atavistic developments or to picture them clearly, the scientists said, but modern scanning technology makes it possible to do so in 3D in unprecedented detail.

Dr Diogo added: ‘It used to be that we had more understanding of the early development of fishes, frogs, chicken and mice than in our own species.

‘But these new techniques allow us to see human development in much greater detail.

‘What is fascinating is that we observed various muscles that have never been described in human prenatal development, and that some of these atavistic muscles were seen even in 11.5-weeks old fetuses, which is strikingly late for developmental atavisms.’

The research was published in the journal Development.

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