The Spleen, Once Regarded as Vestigial, Now Recognized as a Critically Important Organ

Introduction

Claims that progress in the field of anatomy and medicine has been impeded by Darwin’s idea of useless organs that he called rudimentary organs. Sarker et al. opined:

According to the evolutionary knowledge of comparative anatomy, many organs in the human body can be categorized as vestiges. Vestigial in that sense that the organ seemingly has no functions or useless now but they might have functions previously which are lost someway due to evolution. (Sarker et al. 2015, 11)

Sarker concluded that before an organ is labeled vestigial careful evaluation is required to determine if it has a function. As one review correctly noted, the
Appendix, tonsils, various redundant veins—they’re all vestigial body parts once considered expendable, if not downright useless. But as technology has advanced, researchers have found that, more often than not, some of these “junk parts” are actually hard at work. (Koerth-Baker 2009)

One example that illustrates this trend is the spleen, an organ which research has now proven to be critical to good health. The spleen is part of the lymphatic system that, with rare exceptions, exists in all vertebrates (Marieb and Hoehn 2017, 667). As a result of damage from sports injuries, automobile accidents or disease, it sometimes must be surgically removed (Martin 1995, 790). Overall, with proper remedial care, most splenectomy patients can usually live a fairly normal life. For this reason, “for most of medical history, the spleen was considered nonessential to life.” (Nathan 1981, 888).

In his classic documentation of the close-to-100-claimed human vestigial organs, Wiedersheim included the spleen because, in the “placental mammals the lobes are increasingly reduced” and “in the primates, the posterior lobe [of the spleen] has almost disappeared” showing, he argued, as we go up the evolutionary ladder from the lower to the higher animals, the spleen is gradually disappearing (Wiedersheim 1895, 186). In man, Wiedersheim claimed, the posterior lobe of the spleen has almost totally disappeared due to evolution. Many early anatomy authorities, such as Wilder, mention the spleen, but do not discuss its function (Wilder 1909, 293, 363). In contrast, some modern authorities at least speculate on its possible functions (Schmidt-Rhaesa 2007). Likely the reason is because, until recently, it was considered nonessential (Schiller 1991, 205). Some early anatomists, though, recognized hints of the spleen’s actual functions. Kingsley noted “Recent studies tend to show that the spleen may play an important part in the formation of anti-bodies and thus has a value in resistance to infection. It may also have an indirect connection with the formation of erythrocytes” (Kingsley 1926, 348).

Description of the Spleen

The human spleen is a fist-shaped, purple-colored organ about 10 cm (4 in) long located on the upper-left side of the abdomen behind the stomach. The splenic artery carries blood into the spleen, and the processed blood exits by the splenic vein. The healthy adult spleen weighs around 200 g but certain medical conditions can cause splenomegaly, an enlargement that indicates great strain on the spleen. An enlarged spleen results from attempts to deal with infections, such as malaria, liver disease and certain blood cancers (Schiller 1991). Glandular fever (infectious mononucleosis) can also enlarge the spleen, and, if extreme, can lead to spontaneous rupture.

The spleen is composed of a reticular connective tissue framework surrounded by a fibrous connective tissue capsule. Inside the capsule is a complex network of vessels and compartments separated by discrete trabecular partitions (fig. 1). The interior of the spleen contains two tissue types, white pulp and red pulp. The white pulp contains clusters of white corpuscles containing lymphocytes and macrophages and the red pulp has thick masses of erythrocytes (Bowdler 2002, 12,13). Because spleens contain large amounts of blood, they are relatively vulnerable to injury.

Although the spleen is effectively protected by the rib cage, injuries can be serious. Fractured ribs, such as from a car accident or playing football, can puncture the spleen. Because the spleen is highly vascular, this puncture can cause bleeding out into the abdomen, resulting in large amounts of blood leaving the circulating bloodstream. This bleeding can be life-threatening, and, consequently, surgery to remove the spleen (splenectomy) is often done in this emergency.

When the spleen is injured, cells from the
damaged organ can scatter throughout the abdomen.
If the cells land in locations with many blood vessels,
they can begin to grow into tiny extra spleens
called splenunculi (a process called splenosis). The
functional importance of such splenuculi is uncertain
(Rashid 2014).

If the spleen is removed surgically, another organ
(often the liver) will usually take over some or many
of its functions (Tischendorf 1985, 145). Nonetheless,
for reasons not yet fully understood, a splenectomy
drastically shortens the lifespan in a small number
of patients. In most of these particular patients, all
of its major functions cannot be assumed by another
organ after the spleen has been removed (Bowdler
2002).

Functions of the Spleen

The functions of the spleen primarily relate
to circulation (maintaining blood cells and blood
volume) and immunity (filtering blood and housing
immune cells). One of the earliest functions of the
spleen is in the fetus, where it manufactures red
blood cells and blood platelets used for blood clotting.
One reason the spleen was labeled vestigial by some
Darwinists, at least for red corpuscle production,
is because it ordinarily does not fulfill these blood-forming
functions in adults (King and Showers 1964,
303). Recently it has been documented that, in cases
of severe hemorrhage or extreme anemia, the spleen
actually resumes the function of blood cell synthesis
until the emergency has passed.

The spleen also participates in the removal of red
blood cells and blood platelets that are damaged,
defective, and worn-out. The average lifespan of
red blood cells is about 120 days. They then must
be broken down and the parts recycled or disposed;
this job is primarily performed by macrophages in
the spleen. The spleen can also help stabilize blood
volume after hemorrhagic shock causes large blood
losses, for example in a bad car accident, by releasing
blood back into the circulatory system.

One of the spleen’s major known functions is
to help to locate and fight infections. The spleen’s
evaluation strategies involves blood slowing
down as it passes through the spleen, giving
the immune system more time to recognize and
produce antibodies (Hammerquist et al. 2016). This
process is an important part of an effective immune
response that can kill bacteria before the infection is
noticeable. If a person has had their spleen removed,
is born without a spleen (asplenia), or diagnosed
with a low-functioning spleen (hyposplenism), they
have a significant increased lifelong risk of bacterial
infections. The most common bacteria that cause
infections in asplenic people are Pneumococcus,
Meningococcus, and Haemophilus influenzae type B
(Hammerquist et al. 2016, 220). These bacteria are
transferred from person to person through saliva or
mucus droplets from a cough or sneeze. In healthy
persons, this type of bacteria rarely causes illness.

The spleen is uniquely designed to identify
“stealthy” bacteria that are encapsulated by a difficult
to detect polysaccharide coating. Once a patient has
lost their spleen, it is recommended that they receive
immunizations for these encapsulated bacteria, some
of which were named above.

Research on mice has discovered that the
spleen stores monocytes, a type of white blood
cell essential both for immune defense and tissue
repair. This is especially important in microbial
infections, myocardial infarctions, or gaping wounds.
Monocytes are produced in red bone marrow, and
they then travel through the bloodstream to where
they are needed in the body. New research has found
evidence that the spleen contains ten times as many
monocytes as the blood, thus is a far more important
monocyte storehouse than previously thought.
Monocytes are recruited and sent to tissue injury
sites and also trigger inflammation to help control
the infection. Monocytes are also important to aid in
tissue repair. The researchers concluded that “bona
fide undifferentiated monocytes reside in the spleen
and outnumber their equivalents in circulation. The
reservoir monocytes assemble in clusters in the cords
of the subcapsular red pulp and are distinct from
macrophages and DCs (dendritic cells).” They added
that, in response to ischemic myocardial injury, splenic
monocytes increase their motility, exit the spleen
en masse, accumulate in injured tissue, and
participate in wound healing. These observations
uncover a role for the spleen as a site for storage
and rapid deployment of monocytes and identify
splenic monocytes as a resource that the body
exploits to regulate inflammation (Swirski et al.
2009, 612).

Surviving a heart attack requires healing that
depends on monocytes, thus the critical role of the
spleen in heart attack recovery. The researchers also
found from evaluations of lab mice that the spleen is
the source of close to half of the monocytes involved
recovery after a heart attack.

Swirski, the lead author of one spleen study,
wrote: “It was thought that the monocytes that
accumulated immediately after a heart attack were
ones that had been circulating in the blood. But we
did calculations and found that the number that
accumulated in the heart far exceeded the number in
circulation . . . in studies where we removed the spleen
and then induced a heart attack, we saw a vastly
fewer number of monocytes accumulate” (quoted in
Koerth-Baker 2009).

In short, the mice without spleens were not able
to recover from the heart attack nearly as well, if
at all. Now we understand that humans who lack a
functional spleen likewise do not recover nearly as
well.

A 1977 study published in the medical journal,
The Lancet, followed the health of 740 World War II
veterans over 20 years. The study compared veterans
with and without spleens, mostly spleens lost due to
war injuries. The spleenless men were twice as likely
to die from ischemic heart disease and/or pneumonia
and were also slightly more likely to die of liver
cirrhosis (Robinette and Fraumeni 1977, 127). One
reason they were more likely to die of pneumonia is
that they lacked the ability to identify, and to destroy,
the encapsulated bacterium Pneumococcus.

Other Functions of the Spleen

Since the spleen houses large numbers of white
blood cells, it is also an important source of immune
proteins—such as opsonins, properdin convertase,
and tuftsin—produced by those cells. Opsonins are
molecules that enhance phagocytosis by marking
antigens on enemy cells to increase the body’s
protective immune response; they also mark dead
cells for recycling, thereby helping to remove detritus
from the body. Properdin convertase, or Factor
P, is a protein family that activates other proteins
to carry out their role of protecting the body, while
tuftsin is a tetra-peptide that has an immune-stimulatory
effect.

These systems are all critical to overall health, and
consequently those who lack a functional spleen are
more likely to die of, not only heart disease, but also
pneumonia and other diseases. Professor Swirski
noted that we knew the spleen played an important
role in the body, but we didn’t know specifically how
it accomplished its wonders. Now we know how
(Koerth-Baker 2009).

Useless Organs Is Dangerous Logic

Dr. Jeffrey Laitman, Director of the Department of
Anatomy and Functional Morphology at the Mount
Sinai School of Medicine observed that the vestigial
organ idea birthed by evolution is dangerous both to
health and knowledge because history
is littered with body parts that were called “useless”
simply because medical science had yet to understand
them. People say, You can remove it and still live.
But you have to be careful with that logic. You could
remove your left leg and still live. But whenever a
body part is moved or changed, there’s a price to pay.
(Koerth-Baker 2009)

This is very wise advice, as the history of our
understanding of the spleen reviewed above has
documented. The spleen has moved from a useless
organ that is gradually shrinking as we go up the
purported evolutionary ladder from lower to higher
animals, to a critical player in maintaining good
health and helping our body heal.

The Spleen’s Presence in Animals

Although its morphology and physiology varies
in different animals, the spleen is found in all
jawed vertebrates and in no other animal. In most
vertebrates, the spleen continues to produce red
blood cells throughout life; only in mammals is this
function lost in middle-aged adults. In cartilaginous
and ray-finned fish, the spleen consists primarily
of red pulp. It is somewhat elongated in ray-finned
fish because it is located inside the intestinal serosal
lining. Reptiles, birds and mammals contain large
amounts of white pulp. In many amphibians, such
as frogs, it assumes a more rounded form and often
a greater quantity of white pulp (Romer and Parsons
1986, 450–451).

Birds and mammals typically have a round spleen,
adjusted according to its surrounding organs. Many
mammals have tiny spleen-like structures known as
haemal nodes throughout the body that are presumed
to have the same function as the spleen. The aquatic
mammalian spleen differs in minor ways from those
of land-dwelling mammals, such as in general they
are bluish in color. In cetaceans and manatees, the
spleens tend to be very small, but in deep diving
pinnipeds, the spleen is massive in order to store
large numbers of red blood cells that allow them to survive their long stays under water. Furthermore, no evidence exists of a splenic evolutionary transition
between vertebrates and invertebrates. Vertebrates
that are less hygienic, like dogs and cats, have spleens
that are proportionately much larger than human
spleens. This alteration by our Creator enables
scavengers to consume bacteria infested food sources
and, because of the added immune surveillance, to
evade infection.

Summary

The spleen, once regarded as a useless vestige by
many evolutionists, has now been documented to
have numerous important roles in the body, including
recycling red blood cells, filtering blood, and housing
immune cells such as lymphocytes and monocytes as
well as serving as a reservoir for monocytes or plasma
that may be needed elsewhere. The useless organ
idea has impeded looking for the actual function of
numerous organs and structures, and the spleen is
just one more example. All of the formerly claimed
vestigial organs have now been confirmed to have
one or more functions (Bergman 2019).

The spleen is a critical organ in all jawed
vertebrates and is not present in any invertebrate.
Furthermore, no evidence exists for its evolution
from a non-spleen. The available evidence suggests
that the earliest jawed vertebrates had functional
spleens identical to those existing in their modern
counterparts. Research continues to reveal, “the
spleen is an amazing organ that clearly shows the
wisdom and handiwork of our creator” (Menton 2011,
69).

References

Bergman, Jerry. 2019. Useless Organs: The Rise and Fall of
the Once Major Argument for Evolution. Tulsa, Oklahoma:
Bartlett Publishing.

Bowdler, Anthony. 2002. The Complete Spleen: Structure,
Function, and Clinical Disorders. 2nd edition. Totowa, New
Jersey: Humana Press.

Hammerquist, Rhonda J., Kimberly A. Messerschmidt,
April A. Pottebaum, and Thaddaus R. Hellwig. 2016.
“Vaccinations in Asplenic Adults.” American Journal of
Health-System Pharmacy 73, no. 9 (May): e220–e228.
https://doi.org/10.2146/ajhp150270.

King, Barry G. and Mary J. Showers. 1964. Human Anatomy
and Physiology. Philadelphia, Pennsylvania: W. B. Sanders.

Kingsley, John Sterling. 1926. Comparative Anatomy of
Vertebrates. 3rd ed. (Revised). Philadelphia, Pennsylvania:
P. Blakiston’s Son & Co.

Koerth-Baker, Maggie. 2009. “Vestigial Organs Not So Useless
After All, Studies Find.” National Geographic News, July 30.

Marieb, Elaine N., and Katja Hoehn. 2017. Anatomy and
Physiology. San Francisco, California: Pearson Publishing
Company.

Martin, Frederic. 1995. Fundamentals of Anatomy and
Physiology. Englewood Cliffs, New Jersey: Prentice Hall.

Menton, David. 2011. “The Mysterious Spleen.” Answers
Research Journal 6, no. 4 (October 1): 68–71. https://answersingenesis.org/human-body/the-mysterious-spleen/.

Nathan, David, and Frank Oski. 1981. Hematology of Infancy
and Childhood. Vol. 2. Philadelphia, Pennsylvania:
Saunders.

Rashid, Sameeah Abdulrahman. 2014. Accessory Spleen:
Prevalence and Multidetector CT Appearance. Malay
Journal of Medical Science21, no. 4 (July): 18–23.

Robinette, C. Dennis, and Joseph F. Fraumeni Jr. 1977.
“Splenectomy and Subsequent Mortality in Veterans of the
1939–45 War.” The Lancet 310, no. 8029 (July 16): 127–129.

Romer, Alfred Sherwood, and Thomas S. Parsons. 1986.
The Vertebrate Body. Philadelphia, Pennsylvania: Holt-
Saunders International.

Sarker, Aniruddha, Anubha Saha, Sanchita Roy, Santanu
Pathak, and Shyamash Mandal. 2015. “A Glimpse
Towards the Vestigiality and Fate of Human Vermiform
Appendix—A Histomorphometric Study.” Journal of
Clinical and Diagnostic Research. 9(2): (February) AC11-
AC15.

Schiller, Medad. 1991. Pediatric Surgery of the Liver, Pancreas,
and Spleen. Philadelphia, Pennsylvania: Sanders.

Schmidt-Rhaesa, Andreas. 2007. Evolution of Organ Systems.
New York, New York: Oxford University Press.

Swirski, Filip K., Matthias Nahrendorf, Martin Etzrodt,
Moritz Wildgruber, Virna Cortez-Retamozo, Peter
Panizzi, Jose-Luiz Figueiredo, et al. 2009. “Identification
of Splenic Reservoir Monocytes and Their Deployment to
Inflammatory Sites.” Science 325, no. 5940 (July 31): 612–
616.

Tischendorf, F. 1985. “On the Evolution of the Spleen.”
Experientia 41, no. 2 (February): 145–152.

Wiedersheim, Robert. 1895. The Structure of Man: An Index
to His Past History. Translated by H. M. Bernard. London,
United Kingdom: Macmillan and Co.

Wilder, Harris Hawthorne. 1909. The Story of the Human
Body. New York, New York: Henry Holt and Company.