Both blogs begin with the same concept, but the differences between them are as vast as the distinctions between the cities themselves.
New York is a lot bigger, infinitely more diverse and significantly crazier. On the blog, the New Yorkers tend to be funnier and freakier. The wildly dressed chimeras who hardly rate a glance in New York would stop traffic in Philadelphia.
"Humans of Philadelphia" offers a softer vibe, a more traditional portfolio. There's less melancholy and regret, more celebration, and greater interaction between shooter and subject.
Putnam, a lifelong photographer, shoots people all over the city, some at landmarks like the Reading Terminal Market and institutions like the Mummers Parade, others at cultural events like the Hispanic Fiesta at Penn's Landing.
Some pictures he takes while on his way to somewhere else, struck by a person's mood or presence. Many photos seem timeless. His shot of a mariachi singer strumming his guitar looks as if it could have been taken last year or in 1950.
"For me, it's capturing someone's uniqueness," Putnam said. "It could be in their face. It could be in their smile."
"Humans of New York" - HONY for short - is the three-year-old creation of Brandon Stanton, a former Chicago bond trader who, after losing his job, decided to move to New York and take portraits of strangers on the street.
"Mom wasn't too happy about that decision," he wrote on his blog.
Detroit to Vanuatu
But it worked. Stanton has shot more than 5,000 New Yorkers and amassed more than a million followers on Facebook and Tumblr. His blog-based book, Humans of New York, will be released Oct. 15, and last week, it was the fourth-best-selling book on Amazon.com.
A recent HONY photo showed a sad-faced man staring into the distance.
"I kissed a woman yesterday," he said, looking as if he'd not kissed anyone in years.
Another shot showed a red-haired boy, perhaps 10, who was asked, "What's something that adults do that they shouldn't do?"
"Punish kids," the boy answered. "And war."
HONY's influence has spread far beyond New York, its premise mimicked across the nation and around the world. Dozens of "Humans of . . ." blogs have sprung up in cities like Seattle, San Francisco, Chicago, and Detroit, in Houston, Helsinki, and Hungary, in Tel Aviv and Tehran. There's even a "Humans of Vanuatu," a tiny island nation in the South Pacific.
"The more people care about something, the more likely they are to share it," said Jonah Berger, an associate professor of marketing at the Wharton School and author of the book Contagious: Why Things Catch On.
Why has the "Humans" movement multiplied? Two main reasons, Berger said. One is the sheer emotional appeal of the idea. The other is it can be easily replicated.
If the blog dealt with a more narrow topic - say, high fashion, or endangered species - a relatively small number of people would have the expertise to re-create the concept. But practically anyone can photograph people in their community.
Of course, some are better at it than others.
When Putnam first noticed HONY, "I emailed Brandon in New York: 'Can I start a Humans of Philadelphia site?' He said, 'Sure, have fun.' "
Putnam knew that although he could reproduce the original intent, he couldn't duplicate the New York blog nor the flamboyance of some of its subjects.
"There's no city like New York," he said. But the goal of "Humans of Philadelphia" is "revealing the breadth of the character of the city, not just the bizarre things."
Putnam, like his blog, is a combination of forces and factors. He's a Southerner by birth, a retired computer programmer by profession, an Army veteran by the draft, a devotee of Argentine tango by choice. Above all, he's someone who cares deeply about issues of women's rights, race, and the environment - and the need for people to act to make the world a better place.
Putnam lives in a 12th Street rowhouse with his partner, retired psychotherapist Jim Lynch. He's a facilitator for the Awakening TheDreamer Symposium, and a training leader in the symposium's parent organization, the Pachamama Alliance in San Francisco. The group seeks to inspire people to create a sustainable, just, and spiritually fulfilling world.
People on his blog include a young man named Joe, who has a brilliant smile and long braids. His advice: "The world gives back what you put into it. So, you should be happy and try to make others happy."
Another is a woman named Dawn, pictured in a bejeweled top hat as she sells items from her Metal di Muse boutique. "I like to bring out the inner diva in everyone," she said.
What has Putnam noticed about his subjects? "The wisdom," he said. He points to a photo displayed on his laptop computer.
"I love this shot, because of the diversity," he said. The picture shows four friends - from Indonesia, India, London, and San Francisco - drinking coffee in a Rittenhouse Square cafe. "This is the world. But it's also Philadelphia."