Humans Are Striving To Increase Their Lifespans
Life expectancy
Life expectancy or average time humans are expected to live at the time of birth is increasing. As per World Bank, it increased from around 69 years in 2005 to 72 years in 2017, approximately 1 year in every 4 years. It is due to control over diseases e.g. Small Pox (officially eradicated), Malaria, Polio etc; knowledge of balanced diet and availability of the same especially in developed countries; hygiene and clean drinking water etc. Predictably, the increase is less dramatic in developed countries e.g. in US and UK, it increased by 1 and 2 years only respectively in that period. Also this will continue to rise as more countries get developed and benefit from the above mentioned factors.
As per WHO, between 2015 and 2050, the proportion of the world's population over 60 years will nearly double from 12% to 22%. There are more centenarians living today than ever before. The U.N. estimates there were 343,000 centenarians worldwide in 2012, a figure projected to grow to 3.2 million by 2050. However, the question still remains of the limit of human lifespan. The current record is held by Jeanne Calment who died at 122 years.
Other animals and plants
Many animals live longer than humans. Among vertebrates, who have covered spinal chords like humans, Greenland sharks are the longest living animals. They have lifespan of 200 years, one individual lived for around 400 years. Ocean Quahog, a type of mollusc have lifespan of 400 years and one individual lived for 507 years. It actually died when researchers opened up the shell to calculate its age. A jellyfish, Turritopsis dohrnii, converts back to polyp form as it ages which can spawn new genetically identical cloned jelly fish and has no definite lifespan.
This cloning is also the reason for some of plants living for 1000s of years. A Pando clone of quaking aspens in Utah has an estimated age of 80,000 years with trunks connected by a common large root. King’s Lomatia in Tasmania is estimated to be 43,600 years old though each clone has genetically identical but a separate root.
Ageing
The basic lacunae in increasing lifespan is that reasons of ageing are not properly understood and there are multiple theories and factors involved.
Evolution would favour ageing and death else with each arrival of offspring the population will increase sapping all resources after some time. The evolutionary senescence theory of aging says that natural selection is only relevant at reproduction and not at later stages of life and so cannot eliminate late life harmful genes. For example, one gene, P53 prevents cancer in younger age by prohibiting damaged cell to reproduce but can stop body’s ability to renew deteriorating cells in old age.
As cells reproduce they lose a part of their end caps called telomeres and after telomeres are reduced to a certain length, cells stop reproducing. Hence length of telomeres decides the Hayflick limit of number of possible cell divisions. Studies on mice with increased telomere lengths show that they lived longer.
In mice, experiments that reduce food intake by 1/3rd but provide nutritionally balanced food have increased lifespans by 50%. This technique is called Caloric restriction though the reasons of its effect are not clear.
Some of the DNA of our body is found in Mitochondria who help in energy production of the cells. They produce Reactive Oxygen Species or ROS or free radicals in the process which can damage the DNA of nearby Mitochondria. Less food implies less ROS and less damage and this could possibly explain the Caloric restriction theory.
Some research has focussed on the effect of a particular gene and mutating it to see the results. In a nematode, Caenorhabditis elegans, mutating 2 genes, age-1 and daf-2 increases their lifespan. The mutation of the latter doubled the lifespan. Similar experiments in mice have increased the lifespan by 50%. A decrease in Growth hormone/Insulin-like Growth Factor 1 (IGF-1) signalling pathway is shown to increase lifespan in mice and nematode and above mentioned genes regulate IGF-1.
However, experiments and findings on mice and other animals still need to be extrapolated to humans.
Human Genome
Human Genome Project completed in 2003, sequenced or read 4 constituent nucleotides i.e. A (Adenine), T(Thymine), C(Cytosine) or G(Guanine) in human DNA. Today, it is possible to sequence genome of an individual in a few days and it costs a few thousand dollars. The sequencing has many benefits in the field of medicine as it allows us to understand risk of a particular disease for an individual based on his or her genome. This allows preventive care besides creating customized treatment after the onset of disease, thus increasing effectiveness and decreasing side effects. Another project, Earth BioGenome Project (EBP) aims to sequence all living organisms on the earth.
The study of genomes of humans and other animals who live long can help identify genes that directly impact lifespans.
CRISPR
CRISPR or CRISPR-Cas9 is a gene editing technology. CRISPR is a DNA strand used to locate a section of DNA while the enzyme Cas9 is used for editing at that location. While it has applications in industry and agriculture, in humans it can be used for correcting genetic defects e.g cystic fibrosis or Fanconi anemia. Researchers have used CRISPR to correct heart disease defect in embryo or sickle cell anemia in an adult. It can also be used to alter mosquitoes so that they do not transmit diseases.
It also has side effects e.g. it may end up editing only some of the targeted cells, create an imprecise cut or create off target mutations. However the technology is advancing rapidly and new variants e.g. CRISPR-Cpf1 are now available.
Organ Growth
Advances are happening in tissue engineering. University of Texas Medical Branch have developed and transplanted artificial lungs. Wake Forest University has done the same with bladders. Similarly, cartilages have been produced and transplanted. In near future, other organs e.g. bone, skin, pancreas and even blood may be artificially produced. This can help cure the cases of organs damaged or otherwise functioning weakly.
Conclusion
Maximum human lifespan will continue to increase slowly helped by new drugs discovery and surgical procedures, improvement in understanding of balanced diet, awareness of physical activity and hygienic living conditions.
However, understanding of ageing process in other animals and humans, creation and study of genomes and improvement in gene editing technologies and growth in organ development can dramatically change lifespans. Then, humans may face a more difficult question, whether they want to break the limitations of their lifespans.
