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If we evolved to have multiples all the time we would more likely have extended families living together in 1 house. This would be better than the spouses by themselves for multiple reasons including money.

And during prehistoric times it would have been better for several reasons including that 1 would be more likely to survive than if it was born without any siblings.

Most mammals have litters of at least 2. An extreme of this is the cheetah which has litters of up to 9 cubs. They evolved to have litters because they were in conditions where 1 is likely to starve. Lions evolved this for a different reason since lions are not likely to starve. They evolved it because of the fact that they live in prides.

So for primates including us wouldn't it be better if they had litters often instead of single babies?

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  • $\begingroup$ No. Primate babies require a lot of care, as a significant amount of development occurs post-partum. Even though, say, a lion cub requires care and teaching, they are born mobile and with the ability to see and hear fairly well, if not very well. Human babies, on the other hand, are completely helpless. Please remember that money is an extremely recent invention in the span of human evolution, and plays no selective pressure whatsoever. Families choosing to have a limited number of children are also an incredibly recent phenomenon (really, in the last 100 years or so). $\endgroup$ – MattDMo Sep 8 '15 at 16:10
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    $\begingroup$ This is not a duplicate. The answer here is not simply "because not everything advantageous will evolve", this question involves the classic tradeoff between litter/offspring size and the energy expended to care for them. It would not actually be beneficial for primates to have larger litter sizes. $\endgroup$ – terdon Sep 8 '15 at 16:19
  • $\begingroup$ @terdon I am reopening this. If you have an answer for this question then please post it. $\endgroup$ – WYSIWYG Sep 9 '15 at 6:26
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There is a tradeoff between having multiple progeny, with relatively low parental investment in each, and few progeny, with high investment in each. This used to be called "r" and "K" selection respectively, though the terms are now used less. Very simplistically speaking, long-lived species within a fairly stable environment tend to produce few offspring and invest large amounts of care in each; this description fits primates very well, and so it's not surprising that they follow this paradigm. The bottom line is that because of primate life history, having small litters is a more successful evolutionary strategy.

The Wikipedia article on r/K selection theory is reasonably good and points to updates and some further reading on the subject.

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  • $\begingroup$ But some big cats have long lives and large litters. Lions are a great example because lions live for up to 15 years in the wild and have litters of up to 6 cubs. Likewise some animals with short lives have small litters. So the r/K selection theory isn't completely accurate. $\endgroup$ – Caters Oct 11 '15 at 17:33
  • $\begingroup$ The r/K selection theory is not completely accurate (as I noted, the terms are being used less; and the link I included points out the same thing), but lions are not a good counterexample. For one thing, lions are often used as examples of K strategists -- the average litter size is closer to 2 than 6, and a typical interval between litters is closer to two years than one. Three or four offspring every couple years is certainly K-strategy, since with r strategists we're often talking about thousands (or millions) of offspring per year. But as I said, and repeat, K/r is oversimplified. $\endgroup$ – iayork Oct 11 '15 at 20:36

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