Comparative Senescence and Lifespan

Abstract

The word senescence is derived from the Latin word “senex” (meaning old). In biology, senescence is a process by which a cell ages and permanently stops dividing. Senescence is a natural universal phenomenon affecting all living organisms (e.g., humans, animals, and plants). It is the process of growing old (aging). The underlying mechanisms of senescence and aging at the cellular level are not fully understood. Senescence is a multifactorial process that can be induced by several stimuli including cellular stress, DNA damage, telomere shortening, and oncogene activation. The most popular theory to explain aging is the free radical theory. Senescence plays a role in the development of several age-related chronic diseases in humans (e.g., ischemic heart disease, osteoporosis, and cancer). Lifespan is a biological characteristic of every species. The lifespan of living organisms ranges from few hours (with mayfly) to potential eternity (with jellyfish and hydra). The maximum theoretical lifespan in humans is around 120 years. The lifespan in humans is influenced by multiple factors including genetic, epigenetic, lifestyle, environmental, metabolic, and endocrine factors. There are several ways to potentially extend the lifespan of humans and eventually surpass the maximum theoretical lifespan of 120 years. The tools that can be proposed include lifestyle, reduction of several life-threatening diseases and disabilities, hormonal replacement, antioxidants, autophagy inducers, senolytic drugs, stem cell therapy, and gene therapy.

1. Introduction

Senescence is a process by which a cell ages and permanently stops dividing [1]. It is a natural universal phenomenon affecting all living organisms (e.g., humans, animals, and plants). Senescence in cells occurs with the process of growing old (aging). It is a multifactorial process that can be induced by several stimuli including cellular stress, DNA damage, telomere shortening, and oncogene activation.

Lifespan is a biological characteristic of every species. The lifespan of living organisms ranges from few hours (with mayfly) to potential eternity (with jellyfish and hydra). The maximum theoretical lifespan in humans is around 120 years. The lifespan in humans is influenced by multiple factors including genetic, epigenetic, lifestyle, environmental, metabolic, and endocrine factors [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26].

Lifespan can potentially be extended by mutations or by a variety of interventions in humans, animals, and plants. In humans, interventions such as lifestyle, reduction of several life-threatening diseases and disabilities, hormonal replacement, antioxidants, autophagy inducers, senolytic drugs, stem cell therapy, and gene therapy can potentially help to extend lifespan and eventually surpass 120 years [6, 7, 8, 12, 13, 14, 15, 17, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41].

This chapter presents a comparative overview of senescence and lifespan in humans, animals, and plants.

2. History of life on planet Earth

According to the core accretion theory, planet Earth formed around 4.54 billion years ago (approximately one-third the age of the universe) by accretion from the solar nebula [42]. The initial Earth was a dry planet, without atmosphere and ocean components [43].

The origin of life question is one of the most challenging questions in science. Both ribonucleic acid and peptides played key roles in the emergence of life on Earth [44]. Although the beginning of the presence of life on Earth cannot be determined with accuracy, there is evidence that primitive life with bacteria-like organisms was present around 3.50 billion years ago (Figure 1).

Figure 1.

Primitive life was likely present on Earth around 3.50 billion years ago. Copyright symkin (Николай Бибик)/Depositphotos Inc.

The evolution of species is a complex phenomenon that is supported by several theories. According to Darwin, the natural selection enabled simple life to evolve into complex life over a long period of time (Figure 2) [45].

Figure 2.

Evolution of humans. Copyright YAYImages (Alex Golke)/Depositphotos Inc.

3. Diversity of life on planet Earth

Diversity of life refers to the variety of existing living organisms in relation to their species, races, ethnicities, and habitats.

3.1 Species, races, and ethnicities

There is a variety of species (several million), races, and ethnicities on planet Earth. However, most species, including those living in the ocean, remain to be described [46]. Some species are extremely abundant, while others are moderately common or rare.

3.1.1 Humans

The historical definition of human races has evolved over more than two centuries. The initial definition of Black, Brown, Red, White, and Yellow races was at the origin of prejudice and racism. These shameful attitudes are still present in our societies.

To encourage diversity and promote uniformity and comparability of data on race and ethnicity, the National Institutes of Health in the USA has defined the following racial and ethnic categories: American Indian or Alaska Native, Asian, Black or African American, Hispanic or Latino, and White. To be comprehensive with this classification, we should also include different combinations of the above races and ethnicities (Figure 3).

Figure 3.

Diversity of humans. Copyright macrovector (Alejandro Sánchez Blanes)/Depositphotos Inc.

3.1.2 Animals

It is estimated that there are over 7 million animal species, but most have not been described (Figure 4). With approximately 1,000,000 described species, insects represent about 55% of all known species.

Figure 4.

Diversity of animal species. Copyright Jim_Filim (Iakov Filimonov)/Depositphotos Inc.

The basic animal classes include invertebrates, fish, amphibians, reptiles, birds, and mammals. Within each animal species, there are several breeds. For dogs, for example, more than 300 breeds have been described.

3.1.3 Plants

There are close to 400,000 species of plants. The large majority of plants are vascular and flowering plants (Figure 5).

Figure 5.

Diversity of plant species. Copyright interactimages (Matthew Cole)/Depositphotos Inc.

3.2 Habitats

There are two main types of habitats for the living organisms: terrestrial (e.g., regular land, grassland, forest, desert, mountain, and polar region) and aquatic (freshwater, marine, and coastal region). Based on their habitat, living organisms can be exposed to a variety of air, sunlight, temperature, humidity, and noise (Figure 6).

Figure 6.

Organisms living in the water. Copyright Vlad61 (Владимир Голубев)/Depositphotos Inc.

4. Senescence and aging

The word senescence is derived from the Latin word “senex” (meaning old). In biology, senescence is a process by which a cell ages and permanently stops dividing. In addition to exiting the cell cycle, senescent cells undergo other phenotypic alterations including metabolic reprogramming, chromatin rearrangement, and autophagy modulation [1]. Senescence is a natural universal phenomenon affecting all living organisms (e.g., humans, animals, and plants). However, there is a wide range of diversity in the pattern of senescence across species. Senescence is the process of growing old (aging) with a progressive deterioration of the cell and organ functioning. Senescence is associated with a decrease in fertility and/or an increase in mortality.

4.1 Mechanisms

The underlying mechanisms of senescence and aging at the cellular level are not fully understood. Senescence is a multifactorial process that can be induced by several stimuli including cellular stress, DNA damage, telomere shortening, and oncogene activation. The most popular theory to explain aging is the free radical theory [47]. According to this theory, continuous, unrepaired oxidative damage of macromolecules constitutes the molecular basis of aging.

4.1.1 Humans

Human senescence and aging result from accumulation over time of genetic, molecular, and cellular damages.

Aging is associated with a gradual, time-dependent, and heterogeneous decline of physiological functions ultimately leading to death (Figure 7). The human body goes through multiple changes including endothelial pro-atherosclerotic changes, an overall decrease in the size of organs, ovarian atrophy, osteopenia, sarcopenia, skin atrophy, and adipose tissue enlargement (non-exhaustive list) [7, 10, 12, 26]. Some of these changes play a role in the development of several age-related chronic diseases (e.g., ischemic heart disease, osteoporosis, and cancer) responsible for increased mortality. Senescence is emerging as a therapeutic target for several diseases.

Figure 7.

Evolution of humans with aging. Copyright cherstva (Olena Cherstva)/Depositphotos Inc.

4.1.2 Animals

Senescence and aging are also present in animals and influenced by multiple factors including genetic and environmental factors [48]. The rate and intensity of aging vary considerably among animals. Interestingly, some animals do not exhibit clear evidence of biological aging (negligible senescence) and have superior resistance to age-related diseases. These animals include ocean quahog clam, Greenland shark, Aldabra giant tortoise, rougheye rockfish, freshwater pearl mussel, and naked mole-rat (non-exhaustive list). At least two animals, jellyfish (Turritopsis dohrnii) and hydra, are considered potentially immortal [49].

4.1.3 Plants

Senescence and aging in plants are associated with a complex deterioration of cellular metabolism that includes loss of chlorophyll, carotenoids, and proteins, and an increase in lipid peroxidation and membrane permeability, leading to a decline in photosynthesis. Multiple factors including phytohormones, sunlight, temperature, and water play a role in plant senescence [50].

5. Lifespan

Lifespan is a biological characteristic of every species. It is determined by a complex interaction between genetic and environmental factors. The lifespan of living organisms ranges from few hours (animals) to few thousand years (animals and plants) or to eternity (animals). Phenotypic plasticity can affect the long lifespan of both animals and plants [48].

5.1 Lifespan by species

5.1.1 Humans

The lifespan in humans is influenced by multiple factors including genetic, epigenetic, lifestyle, environmental, metabolic, and endocrine factors [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26].

The maximum theoretical lifespan in humans is around 120 years. Very few individuals reach this theoretical age since several events can impact lifespan (e.g., diseases, suicide, accident, and war).

Centenarians are subjects living 100 years or older. They represent a model of successful aging [9, 31, 51, 52]. Semi-supercentenarians are those who reach an age of 105–109 years (Figure 8). A very small fraction of centenarians (up to 0.5%) will live 110 years or older (supercentenarians) [5, 52]. The oldest supercentenarian with well-documented age was Jeanne Louise Calment (1875–1997) from Arles (France) who lived 122 years. The second oldest supercentenarian was Sarah DeRemer Knauss (1880–1999) from Hollywood, Pennsylvania (USA) who lived 119 years.

Figure 8.

A semi-supercentenarian Chinese woman (105 years old) and her great granddaughter. Copyright ChinaImages (IMAGINECHINA LIMITED)/Depositphotos Inc.

According to United Nations estimates, in 2020, the number of centenarians in the world was approximately 573,000 (mainly from the USA). This number could reach approximately 3,676,000 by 2050 (mainly from China).

5.1.2 Animals

The lifespan in animals is between few hours to potential eternity. The shortest lifespan is seen with mayfly (1 day). Aside from jellyfish (Turritopsis dohrnii) (Figure 9) and hydra which are considered potentially immortal animals, the longest lifespan has been observed with glass sponge (around 10,000 years) (Table 1) [49].

Figure 9.

Jellyfish (Turritopsis dohrnii) is considered a potentially immortal animal. Copyright Winston (Sergey Belov)/Depositphotos Inc.

Animal	Lifespan
Jellyfish	Potentially immortal
Hydra	Potentially immortal
Glass sponge	10,000 years
Coral	4,000 years
Clam	500 years
Shark	270 years
Giant tortoise	250 years
Rougheye rockfish	200 years
Parrot	100 years
Crocodile	70 years
Elephant	50 years
Monkey	45 years
Camel	40 years
Horse	30 years
Cow	20 years
Snake	20 years
Cat	18 years
Lion	15 years
Tiger	15 years
Dog	13 years
Rabbit	12 years
Chicken	10 years
Pigeon	5 years
Ant	4 years
Rat	2 years
Mosquito	6 weeks
Fly	1 month
Mayfly	1 day

Table 1.

Lifespan (average or record) in animals ranked by descending order of duration (non-exhaustive list).

5.1.3 Plants

The lifespan in plants ranges from few weeks (in annuals) to few thousand years (in trees). Bristlecone pine (Pinus longaeva), a tree found in the higher mountains of California (USA), is among the plants with the longest lifespan (around 5,000 years) (Figure 10).

Figure 10.

Bristlecone pine (Pinus longaeva) can live around 5,000 years. Copyright mkopka (Melissa Kopka)/Depositphotos Inc.

5.2 Evolution of lifespan

The important increase in human lifespan over the past 100 years is one of the greatest achievements of humanity [53]. The evolution of human lifespan over time is reported in Table 2.

Period	Lifespan
Pre-historic	30 years
Early 16th century (developed countries)	40 years
Early 20th century (developed countries)	50 years
Early 21st century (worldwide)	73 years

Table 2.

Evolution of average lifespan in humans throughout the history of mankind.

5.3 Interventions to extend lifespan

Although lifespan is a biological characteristic of every species, it can be modified by mutations or by a variety of interventions in humans, animals, and plants.

Extending lifespan while keeping health and vitality has always been a dream for mankind. The “successful aging” is aging without any disabilities and severe diseases (Figure 11) [27, 53].

Figure 11.

Healthy old couple. Copyright nicoletaionescu (Nicoleta Ionescu)/Depositphotos Inc.

There are several ways to potentially extend the lifespan of humans and eventually surpass the maximum theoretical lifespan of 120 years. The tools that can be proposed include lifestyle, reduction of several life-threatening diseases and disabilities, hormonal replacement, antioxidants, autophagy inducers, senolytic drugs, stem cell therapy, and gene therapy (Table 3) (Figure 12) [6, 7, 8, 12, 13, 14, 15, 17, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41].

Tool	Example
Lifestyle	Diet (rich in vegetables, fruits, and fiber, poor in saturated fat and red meat), Caloric restriction, Exercise, Clean and stress-free environment
Reduction of life-threatening diseases	Treatment of Ischemic heart disease, Chronic obstructive pulmonary disease, Cancer, Overweight/Obesity, Type 2 diabetes, Osteoporosis, etc.
Hormonal replacement	Estrogen, Dehydroepiandrosterone
Antioxidants	Resveratrol, Curcumin, etc.
Autophagy inducers	Caloric restriction, Exercise, Vitamin D, Resveratrol, Metformin, Rapamycin, etc.
Senolytic drugs	Dasatinib, Quercetin, Fisetin, Navitoclax, etc.
Stem cell therapy	Stem cells
Gene therapy	Gene editing, Viral or Non-viral vectors

Table 3.

Tools proposed for the potential extension of the lifespan in humans (non-exhaustive list).

Figure 12.

Regular exercise can be very beneficial for the extension of lifespan. Copyright kornetka (Svetlana Ivanova)/Depositphotos Inc.

Clinical trials using anti-senescent therapies are in progress. Results from early pilot studies suggest that senolytic drugs can decrease senescent cells, reduce inflammation, and alleviate frailty. Stem cell therapy represents a new emerging era in medicine that has the potential to delay the aging process and, therefore, extend lifespan, by better treating life-threatening diseases that impact lifespan. Genetic interventions, although promising, may be difficult to implement in humans without the knowledge of all the potential health consequences during entire life.

6. Conclusions

Senescence is a process by which a cell ages and permanently stops dividing. It is a natural universal phenomenon affecting all living organisms (e.g., humans, animals, and plants). Senescence is the process of growing old (aging). It can be induced by several stimuli including cellular stress, DNA damage, telomere shortening, and oncogene activation. The most popular theory to explain aging is the free radical theory.

Lifespan is a biological characteristic of every species. The lifespan of living organisms ranges from few hours (with mayfly) to potential eternity (with jellyfish and hydra). The maximum theoretical lifespan in humans is around 120 years. The lifespan in humans is influenced by multiple factors including genetic, epigenetic, lifestyle, environmental, metabolic, and endocrine factors. The lifespan in animals is between few hours to potential eternity. The lifespan in plants ranges from few weeks to few thousand years.

There are tools that can potentially extend the lifespan of humans and eventually surpass 120 years. They include lifestyle, reduction of several life-threatening diseases and disabilities, hormonal replacement, antioxidants, autophagy inducers, senolytic drugs, stem cell therapy, and gene therapy.

Conflict of interest

The author declares no conflict of interest.