"And in studies where we removed the spleen and then induced a heart attack, we saw a vastly fewer number of monocytes accumulate."
Simply put, mice without spleens weren't able to recover as well.
Neither, it seems, can humans without spleens.
A 1977 study, published in the medical journal The Lancet, followed the health of World War II veterans over 20 years—some with spleens and some who'd lost theirs to war injuries.
The spleen-less men were twice as likely to die from heart disease and pneumonia.
"They knew the spleen played an important role, but they didn't know how," Swirski said.
None of this is surprising to Jeffrey Laitman, director of anatomy and functional morphology at New York City's Mount Sinai School of Medicine and president-elect of the American Association of Anatomists.
History is littered with body parts that were called "useless" simply because medical science had yet to understand them, Laitman said.
"People say, You can remove it and still live. But you have to be careful with that logic," he said. "You could remove your left leg and still live. But whenever a body part is moved or changed, there's a price to pay."
Appendix Rescued From Biology's Junk Heap
In some cases, life in the developed world—rather than insufficient medical technology—has obscured important functions of vestigial organs.
The appendix, a narrow tube that hangs off one end of the colon, is probably the most famous "junk" organ. But it's turned out to be important even today—in certain circumstances.
"It's hard to figure out what the appendix does when you're studying superclean animals and people," said Bill Parker, assistant professor of surgery at Duke University Medical Center and one of the researchers who exposed the appendix's secrets in a 2007 Journal of Theoretical Biology study.
Far from useless, the organ is actually a storehouse of beneficial bacteria that help us digest food (interactive digestive-system guide).
The appendix evolved for a much dirtier, parasite-plagued lifestyle than the one most people live in the developed world today, Parker said. But where diarrheal disease is common, for example, the appendix is apparently vital for repopulating intestines with helpful bacteria after an illness.
Another example of anatomy lagging behind lifestyle, according to Mount Sinai's Laitman, is collateral circulation. Certain systems of veins and arteries ensure blood flow when the main paths are blocked or damaged.
The systems appear to be truly vestigial, at least for now.
Elbows, knees, and shoulders, for example, all have collateral circulation, Laitman said, but the heart and much of the brain don't.
"Why would we adapt enormous redundancy in an elbow but not where it really matters?" Laitman said. "The answer is unsettling. When do we have strokes and heart attacks? Our 50s, 60s.
"When the blueprints for our species were being drawn up, nobody lived that long."
The fact that our bodies evolved while humans lived short lives hunting and gathering is one key to understanding many "useless" body parts, Laitman said.
From an evolutionary viewpoint, we've been living in the modern manner for a relatively short time, he pointed out. "Our circumstances have changed a lot, but our bodies haven't."
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