How the “junk DNA” led to humans being tailless

News Excerpt:

According to new research a genetic change in our ancient ancestors may partly explain why humans don’t have tails like monkeys.

More about the research:

  • Scientists have traced our tail loss to a short sequence of genetic code that is abundant in our genome but had been dismissed for decades as junk DNA, a sequence that seemingly serves no biological purpose. 
  • They identified the snippet, known as an Alu element, in the regulatory code of a gene associated with tail length called TBXT.

TBXT (T-box transcription factor T) gene:

  • The TBXT gene provides instructions for making a protein called brachyury. 
  • Brachyury is a member of a protein family called T-box proteins, which play critical roles during embryonic development. 
  • T-box proteins regulate the activity of other genes by attaching (binding) to specific regions of DNA. 
  • On the basis of this action, T-box proteins are called transcription factors.

What is a gene?

  • A gene is a segment of DNA that contains sequences of many bases, varying in size from a few hundred to 2 million. 
  • Each gene affects a specific aspect of health. For example, some genes contain instructions on how to make specific proteins.
  • Parents pass on their genes to their biological children. As a result, each person has two copies of each human gene one from each parent.

What is a genome?

  • Every cell of an organism contains a full copy of that organism’s DNA, called the genome. 
  • The genome contains the information that the cell uses to make proteins, the workhorses of the cell.
  • Genome refers to all the genetic material in an organism. The human genome consists of around 3 billion DNA base pairs.
  • Almost every cell in the body contains a complete copy of the organism’s genome, tightly packaged inside its chromosomes. 

Compact Genome:

  • As scientists began to determine the genome sequence of organisms in the mid-1990s they observed that simple organisms like bacteria maintain highly compact genomes.
  • Bacterial genomes exhibit a tandem arrangement of genes, where one gene ends, another begins.
  • Genes constitute a significant portion, approximately 85-90%, of the bacterial genome.

Deoxyribonucleic Acid (DNA):

  • DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. 
  • Nearly every cell in a person’s body has the same DNA. 
  • Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). 
    • Mitochondria are structures within cells that convert the energy from food into a form that cells can use.
  • Our DNA is unique, unless you’re an identical twin.

Junk DNA:

  • In genetics, the term junk DNA refers to regions of DNA that are noncoding.
  • DNA contains instructions (coding) that are used to create proteins in the cell. 
    • However, the amount of DNA contained inside each cell is vast and not all of the genetic sequences present within a DNA molecule actually code for a protein.
  • Today we know this ‘junk’ DNA is responsible for various functions including controlling when to make a protein and when not to.
  • A significant fraction of the ‘junk’ also contains transposable elements. These are pieces of DNA that can shift their positions within the genome.

Alu Element:

  • One such transposable  element, called Alu, is unique to primates (both apes and monkeys). 
  • It is tiny, being made up of around 300 base-pairs (the human genome is approximately 3 billion base-pairs), but has the ability to copy itself and ‘jump’ within the genome.
    • It is present in 1.4 million different locations in the human genome. 

The Alu accident

  • Twenty-five million years ago, after the ape and monkey lineages separated, a chance insertion of an Alu element occurred in an important gene in the zygote of an ancient creature. 
    • It caused that ancient creature to not develop a tail.
  • Because the insertion had happened in the zygote, it was imprinted in the DNA of every cell of that creature, and its subsequent offsprings. That creature was the ancestor of all modern apes.

Key findings of the research:

  • The research team has found the Alu insertion between two pieces of a gene called TBXT – a gene already known as one of many involved in tail formation in monkeys. 
  • As a result of this insertion, apes can’t stitch the pieces together correctly and ultimately produce a TBXT protein with one part missing. 
  • According to the research this insertion was present in all apes and absent in all other monkeys – a strong sign that it’s the cause of tail-loss in apes.
  • The research also determined that the defective TBXT protein caused other problems, including neural tube defects. 
    • It predicts that there must have been compensatory changes to the genome to overcome these defects. 


Despite the excellent work of the New York University (NYU) team, we may never fully understand the tale of our tail. Tail loss has been implicated in bipedalism: our ability to walk on two legs. But it is difficult to speculate on exactly what evolutionary benefit was conferred on the ancestral tailless ape that led to its selection by nature. Whatever that selection pressure may have been, what is incredible is how evolution seized upon that one-in-a-million event and used it to create an ape that would go on to rule the world.

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